The Nikon D800 made the limitations of the OVF very clear in my case, and the liveview was poor. That camera turned out to be my last DSLR. The D700 was nice and 12mp less taxing, but back then Nikon was still Nikon.
I didn't have problems using the D800 for manual focus using the live view, as most of my manual focus work is with tripod and stopped down for large depth of field, but the D810's live view is much better (higher resolution, less noise in lower light, two subsections of the image can be used to adjust tilt and focus in some cases). The D810's OVF is also with better coatings giving a clearer image which more pleasing to work with. The D5 is a further improvement in viewfinder quality in my experience. I don't routinely use it for manual focusing hand held though, since I use autofocus lenses now for that kind of shooting. For the tripod based shooting typically I shoot with the D810 and aim for extended depth of field (stopping down to f/9-11, using tilt to maximize the near-to-far sharpness, I find the OVF useful for initial adjustment of the tilt and focus, to be refined by LCD). If I wanted to use manual focus with fast primes, I too might feel strained eyes, but when trying the A7R II, I noticed that I wasn't able to focus using the unmagnified EVF even remotely well enough to get usable results with the 135/2 Apo Sonnar. By using the zoomed view focusing was made possible but then I can't monitor the subject's and the image's overall expression at the same time if I have to view a zoomed-in subsection of the image. This kind of approach of hand holding the camera and zooming in for a partial view is not an acceptable way of working for me and I wouldn't be able to concentrate of the image content and timing doing that. I by far prefer to use an OVF to monitor the subject's emotion and clues to changes in it, and let the camera's continuous autofocus do the focusing. Subtle details of facial expression is something I can't see through an EVF, and without the use of zooming in, manual focus would not be possible either, so for me the EVF concept is unfortunately a non-starter. It doesn't give me anything that I could use to work with. However, I completely understand that my subjects are not everyone's, and people have widely varying preference regarding viewfinders and what kind of camera design works best for them. This is what is great about the situation today: There is something for everyone.
The D800's AF gave me plenty of headaches with fast primes; the D810 was a significant improvement which solved most of the inconsistencies that I had. The D5 AF is phenomenally fast and sensitive in low light and in particular, it allows me to shoot moving subjects at f/1.4 on a routine basis with very few out of focus images. Especially with longer distances the earlier Multi-CAM 3500 series cameras had problems which I do not see with Multi-CAM 20k. However, time will tell how the new module will perform on a high resolution sensor. For me 20-24MP is plenty enough for most of my practical applications, and I've been very happy with the D5's resulting image quality, though not as good as D810 at low ISO.
In 2012 there is a breakthrough in one area (sensor and image quality) but because of production problems due to the 2011 natural disasters I understand that they had difficulty making correctly functioning cameras. This is regrettable but Nikon's products are far too valuable to me to not forgive them for their errors. I can understand that when there are large-scale casualties, evacuations, no working infrastructure, it can be difficult to concentrate on manufacturing cameras with precision requirements. Things have improved greatly since and I'm very happy with Nikon's quality control from the last few years. Also their progress with the D5 autofocus is really amazing.
