I installed Sierra just a few days ago on an Intel server board (S5000XALR). I had to change a single setting in my bootloader (clover) to make all the services work (Admittedly, it did take a bit of googling to find that setting).

I think Intel boards are ideal because they are presumably quite similar in hardware to the Mac Pro motherboards. All my hardware (including GTX480) worked without any additional configuration.

iMessage working on both my systems, it was not a pain in the ass at all. The only thing I did was make sure my two boxes had unique serial numbers.

I never use it, though guides constantly mention you gotta have the right system profile and some other low level settings for it to work.