Well, if you don't want too much troubles, go with nvidia, or go with amd series 6000 (could be expensive depending on your budget).
Note that a kernel fix is included in unraid that should fix some older amd gpus, but this may work or not, depending on brand, firmware, revision, etc...
Quick search on google or even here in the forum for "amd gpu reset bug" and you will find a lot of info.
If you are going to buy a second hand nvidia gpu, it should be preferred that updated drivers still exist: this because old nvidia gpus without newer drivers (i.e. with older drivers) are not able to be passed through in a vm, unless you modify the xml to hide the hypervisor.
Only with newer nvidia drivers (by "newer" I mean from v. 465), nvidia allowed for its consumer gpus (geforce, titan) to be passed through in vms.