Upon impact, plastic has the ability to return to its original shape (to within limits) whereas metal, once it is bent, it stays bent.
My experience with the D810 has been very positive, it doesn't have any of the strange AF effects of the D800 that I had. The D810 shows much less colour and distance variability of autofocus error than the D800 which was a nightmare in that respect. E.g. the 105 DC autofocuses very nicely with a fixed -12 to -13 fine tune setting on my D810, whereas it required -20 at infinity and about 0 at close-up distances with the D800, which made its use extremely impractical. Now (with the D810) it works much like any lens, except the skin is so beautiful. I have noticed similar (but less severe) effects with other fast primes (e.g. 85/1.4 AF-S) and so the move to D810 was a delight for me. I also like many other aspects of the camera which have been improved.
I've noticed no AF shift over almost two years (and 60000 frames) of use with the D810; the original fine tune settings that I made after purchase still stand (and they're much smaller than what I needed with the D800). It is also quieter and better shaped for my hands, and the viewfinder shows better contrast in sunny conditions which I'm very happy about as well. The large buffer has also made it practical to use it, e.g., for catwalks, processions and sports without the buffer running out even in hectic shooting.
However, I haven't dropped the D810 so I don't have any data regarding its ability to withstand impacts. I did drop my D800 with 24-70 attached; the AF went haywire a bit which JAS fixed for free. The fact that I haven't noticed any shift in required AF fine tune values in the D810 over time is encouraging however, and I have thought that I should have bought two of those instead of getting the D750 as a second body. The D750 is nice as a small body goes but it's just too small and the grip is too narrow and deep for my fingers. Both the D810 and D750 have been problem free for me. One of the differences that I noticed is that the "focus confirmation dot" on the D810 is quite precise and is possible to use for fine tuning the AF (focus using live view, then turn AF off and go into OVF viewing, go through AF fine tune settings in the menu and check the range of fine tune settings which show the dot, then take a middle value as the new fine tune setting - works without taking a single picture) whereas with the D750 the range of fine tune settings where the focus confirmation dot is seen is too wide to get useful information for fine tuning with many lenses. So with the D750 I'm forced to take test shots to arrive at usable fine tuning values. With the D810 the mean of optimal fine tune settings has been close to zero (I think it's +1 or +2) whereas with the D750 it is around +7. I know these are just one user's findings on one sample of each camera but to me everything that I've seen about the D810 echoes that it is more precisely made than other cameras that I've used, especially regarding the AF. However, once fine tuned the D750's AF has worked well for me and I have no functional complaints about that camera. It's just a bit too small for me to feel comfortable holding and using it. Also I prefer the motor driven, quieter mirror of the D810. I guess the D750 is excellent for backpacking because it takes less space and is lighter than the D810. So there is no one camera which is best for everything and each have their own merits.
Hopefully they will soon make a D810 successor with Multi-CAM 20k; the extended area of cross type point coverage would help me a great deal as I shoot a lot of verticals of people with the focus on the face at the top of the frame, where there are only linear points in the Multi-CAM 3500 cameras. The D5 upon first contact seemed to handle focusing in this area of the frame (far edges of the long dimension of the frame) much better ... but it's a little too rich for me, given that I rarely use CH, preferring manually timed exposures almost always.
I trust Nikon to make whatever improvements they can to improve the camera's impact resistance and long term stability. This is important for AF in a high resolution camera. The D810 has been good to me in that respect but it can be improved I am sure.
