You lose the ability to focus to infinity, which makes it largely useless unless you're shooting macros.
Nikon's live view is just fine. The D810 is much better than the D800 in that regard.
Just to be clear, it's only A mount (and even then you need the appropriate adaptor, of the 4 options) and EF mount lenses which currently have AF capable lens mount adaptors.
That out of alignment is of little concern. There are very few viewfinders out there which show 100% of what the image sensor sees so a misalignment of less than 1% will not be noticed by anybody.
Only when you are talking noise at high ISO sensitivities and then only if you aren't pushing and pulling the raw image.
For sublimesum's sake, you can't mount an adapted EF lens to an F mount (Nikon) body and retain infinity focus - the F mount flange length is longer than that on the EF mount
good to know. thanks