I would be much more apt to choose a 10-series Nvidia GPUs, rather than Polaris or newer on AMD side, if you wanted a card that would boot in a legacy / pre-UEFI rig, or using CSM boot.
I've seen NVidia cards with "hybrid BIOS" work in legacy and CSM boot modes, far more often than AMD GPUs. Sapphire cards in particular seem to be "picky" about having a mobo with a UEFI BIOS in UEFI (non-CSM) boot mode. Conversely, I've heard, but not tested, that PowerColor cards will supposedly work in a legacy board. I don't think that I've tested that, could just be a word-of-mouth rumor, and could be totally incorrect.