UEFI vs BIOS is an open question. I have seen some cards which indicate dual settings compatibility. However, I have seen reports of AMD R series cards not running on an older non-UEFI motherboard. If you have a legacy BIOS motherboard as I do, you should confirm that these cards will actually work with your system.
Non-UEFI compatibility for RX series depends on the AIB as well from what I've heard. For example, Sapphire cards only support UEFI.