Would you rather have too much "Engineered Compliance" or not enough?

I was on the fence a little bit but chose slightly too flexy - part of me worries in the back of my head that more flex could be leading to wear/fatigue in certain components (even if it actually isn't), but then I have had parts which had some definite flex (particularly wheels and handlebars) and those bikes were super fun/comfortable to ride. Changing to excessively stiff parts were definitely more twitchy/uncomfortable. 

Examples - 

Old Dj bike with profile cranks which were stiff and jarring in the ankles, but the 25.4 bars were very smooth upfront (until I changed them to a 31.8 set up, which I regretted)

Trail bike with Enve XC wheels were comfortable with predictable grip (even if the rim/tyre width was too narrow so the ultimate grip wasn't good) but changing to the extremely stiff AM version felt like planks of wood. These days I like a middle ground, and ideally a more flexible wheel up front

The Trust Shout fork - in general the suspension action was amazing and confidence inspiring but the lack of vertical compliance (which DW acknowledged) contributed to harshness/nervousness which undid that confidence

I went with too stiff but with the caveat that's it's slightly.... I can still remember the feel of a 160mm Fox 32 (circa 2015..) twisting and I don't wish to revisit that sensation. 

Having said that I also remember those 1st gen enve wheels where you could feel every pebble so much too stiff is as bad as much too flexy.

And as I get older comfort seems to matter more and more (or all my trails are just getting way rougher....)

I selected too stiff, but as with everything in my dumb ole brain, there are caveats.  

My most recent experience with a bike that was too flexy, was a first generation orbea rise.  That bike had a relatively stiff front triangle and an absolute door hinge of a pivot/rocker link and rear triangle.  It would flex to the point where I could get the rear tire to rub the chainstays and you could feel the rear end flex contributing (and at times, detracting from) cornering and being able to hold a line.  It would be extremely sketchy on some high consequence moves in laguna (The CB Drop on PGs for those that know for example).  That was in contrast to the majority of my experience on Santa Cruz bikes of being slightly too stiff for my riding style and abilities.  

Overall, I think it really depends on what part of the bike we're talking about here.  With wheels, I'd rather bias to slightly more flexy then optimum over being too stiff.  With frames, I'd like it to be slightly stiffer then optimum than too flexy.  Up until recently, I would have not had a strong opinion about forks, but now I believe I want them slightly too stiff rather then slightly too flexy (based on my experience with the nine.one).  In the early days of carbon wheels, when everything was way too stiff, I used to have a rule that carbon wheels would get paired with coil suspension.  

Overall I think it comes down to predictability (or lack there of) as the first indicator of whether flex is beneficial or not.  My experience with the rise and the nine.one was that the flex was unpredictable and as a result, was very hard to adjust to and incorporate into my riding.  In contrast, if the flex has a predictable nature to it and it comes with either added comfort (less perceived harshness) or performance (more grip or more forgiving) then the trade off becomes not only worth it, but welcomed.  

All that being said, I do some kook stuff occasionally in the name of flex.  For example, on my ebike I am running a reserve hd 30 carbon rear wheel and a reserve hd 30 aluminum front wheel.  I also used to build my enduro race and dh race wheelsets with an 32h ex511 rear rim and a 28h xm481 front rim.

Hmmmm. In my mind, I am an absolute specimen who demands absolute perfection and zero flex. Perfect, zeroed performance right down to the millimeter. 
In reality, flex will help you out. Since I am also mindful of the fact that I can't really tell if my shock mount bolts are loose, (true story,) I probably wouldn't notice. 

IDK, I picked the crippled with indecision option, because, well, ...I am. 

Too much. Riding the first version of the Ripmo AF (it was pretty much a noodle) and it was mostly ok other than coming out of really fast steep berms, which is becoming a very small portion of my riding. Off camber, weird slow tech and the like… it was pretty good. Next bike was a carbon Capra MX that was much stiffer and longer days on it were a lot tougher on the body. It was shocking how much worse the Capra was in the mud. 

Too stiff because of the fear of something being too flexy also being unpredictable.

On the other hand there are situations where you want more or less flex. If you've been off the bike for a while and get a bit rusty, a softer bike would be good to help you deal with getting back on the horse. Then getting back up to speed it would make sense to again have a stiffer setup. So it's not one size fits all not just for everybody, but even for one rider at different times. 

And regardless of my answer and fear of things being unpredictable (unintended steering moments), I think we humans can deal with a lot of things if needed. Even a very flexy handlebar, regardless of how people say steering needs to be precise, can fade into the background after a run or two once you get used to things.

I've had issues with products on both ends of the spectrum.  LB carbon rims that were brutally stiff, and when I replaced them I couldn't believe I rode them for as long as I did.  Levo gen3 frame was so flexy that the seatstay bearings would shift every ride.  The reason I ended up voting too much compliance was that I felt like it's easier to compensate for, whereas the only solution to too stiff is "get stronger or suffer harder."  That said, I think that too much compliance has the drawback of shorter product lifespan:  you're going to tear through soft rims, light casing tires, and noodly forks if you ride them hard.

also went with too much stiffness here.  i'm a bigger guy and still ride fairly hard. i also started riding in the mid 90s when bikes and components were often made from spaghetti

Flex!

As Falcon mentioned: "in reality, flex will help you out". So true!

I've been riding a way too flexy, steel prototype. Pretty extreme geometry, so at first I didn't notice the flex at all, since the geometry took all my learning efforts. And after 4 weeks of pretty extreme terrain, there was never a moment that the lack of stiffness made riding harder. Going back to a 'regular' aluminium bike after those 4 weeks was an eye opener. I was all over the place, couldn't hold a straight line anymore, was taking inside lines everywhere and outside lines after slight corrections. Precision failed completely on the regular bike, every input felt amplified, mistakes everywhere. Took me a couple of rides to dial it in again.

Unless you need absolute precision (that is, when you have this absolute precision. And then you're probably called Loic Bruni), a flexing bike makes you a better rider. If you're used to a too stiff, regular bike, it will probably take you a few rides to get the feeling right. But after that, you're a better rider!

If it's too flexy, I struggle to ride the bike confidently.  The bike wiggles in unpredictable ways.  If it's too stiff, I might have a hard time going as fast as I potentially could, but it's not for a lack of confidence or predictability in the machine.  I'll take a bike that's too stiff.  Too flexy also means I'm one hard corner or landing away from over flexing to the point of failure. 

I chose too flexy but I'd imagine this has a lot to do with what your local trails are like.

I destroyed a stans flow rear wheel on my Transition Sentinel and needed a replacement rim but decided to be cheap; I ended up throwing a 2014 Stans Arch Ex rim (21 mm internal width) on the rear wheel that I ran for about 9 months. Besides the fact that it would flex and rub on the chainstay, it felt awesome and super grippy. I only got scared riding it down Telonics where I could feel the poor thing fighting for its life down high-speed turns. Were it not for the rubbing and durability concerns though I might have kept running it.

one aspect I personally think is underrated in “compliance” is not only flex but how well or not well the product dampens vibrations and how quickly/slowly the material “unflexes”.

For example I personally think aluminum does a pretty great job as a handlebar material.  An equally stiff carbon bar may be possible lighter but it’s usually a lot harsher.  Especially at transmitting small bumps.  

That same property makes aluminum frames either flexy or kind of muted and heavy.  Where a carbon frame can be pretty stiff feeling as well as fairly light.  As well as add localized stiffness in a way that doesn’t seem possible with aluminum without adding a bunch of weight.

This feels like a similar conversation to underbiked vs overbiked. I voted too stiff, for the same reason I preferred to be overbiked - I’d rather have the extra safety and security when I’m at my absolute limit. The other challenge is that my terrain varies massively, from natural trails at lower speed, to wide open bike park laps. So for me, too stiff for half my riding ends up being pretty much perfect for the other half. Obviously, for the sake over conversation, it’s been taken to the extreme where all your components are too stiff/too flexy, but in reality you can fine tune the whole assembly to offer some flex in some places, with stiffness in other places to get the best combination for the whole system. 

I've maintained or gained confidence through overly stiff, even if I can get a bit beat up or bounced off line, while flex shows up unexpectedly and steals confidence.

Also, stiffness can sometimes be tuned down through other parts, while flex usually cannot be made more stiff.

Picked more noodle. Compliance seems to be the name of the game.  I also don't ride bike park type stuff as much these days.  For steeper, looser and root filled trails, a bike that is able to do the salmon a bit seems nice.  I also dont have a hard time placing my tires where they need to be, comfort and traction seem to be more important. 

Why do you ask questions I have to ponder the whole day and constantly come back to @TEAMROBOT ?

Slightly too stiff is at least a consistent ride. Slightly too flexy can feel unpredictable. 

It's also easier to add some compliance to a bike with wheels and handlebars, there's really not much you can do with a too flexy frame.