While the D5 does not have very good dynamic range at low ISO, it does have excellent high ISO dynamic range. D850 low to medium ISO images are smoother and more editable, but I've always preferred the D5/6's high ISO tonal and color quality (and now D6's). In practice I have not felt the D5's low ISO DR to be an issue but I've always had other cameras to shoot scenes with high lighting contrast when image quality has been a higher priority than catching the shot.
With regards to AF, the D5 can achieve a higher percentage of focus keepers when photographing approaching subjects with fast primes such as 105/1.4 and 200/2 compared to D850. However, with the 70-200/2.8 FL, I've felt both cameras are excellent. I used the D5 when I really needed to catch the shot with no margin for error. The D6 makes catching the shot especially with human subjects really easy.
The D5 viewfinder is easier to use for action subjects as it is easier to see the frame edges and corners while shooting from slightly awkward angles than the D850 which is a bit tight for me and requires really optimal positioning of the eye behind the ocular to see into corners. During long shoots I find the D5 more comfortable. The D6 viewfinder also has this extra eye relief.
I nowadays use the D850 for landscape, close-ups and similar subjects that don't move and shoot the moving subjects in most cases with the D6, which has a delightful AF system and a lower-pitched sound so enjoy it more. And so many user-friendly features that make shooting easier and more refined (such as a glove mode for touch screen, custom group area with face priority and a high degree of customizability, and connectivity).
For large prints, the D850 delivers impeccably detailed images. As mentioned before, at medium-high to really high ISO, I prefer the outcome from the D5/6.
I have several build quality issues with the D850. My D850's multi-selector is not as tactile-responsive as those of the integrated vertical grip models and sometimes with gloves on, I struggle with it. The vertical grip-body joint is not as rigid as I'd like; this can be an issue for tripod-based macro work. The D5/6 don't have any such problems. Mounting the 90 degree viewfinder attachment to D5/6 is a breeze as one just slides it in (assuming you have an extra eyepiece attachment).