Graphics card/general hardware support for UEFI/Legacy BIOS?

Graphics card vendor websites have BIOS support information- Legacy/UEFI/both.

1) Why does some graphics card vendor mention BIOS support and some totally ignores it?

2) If I have a old legacy BIOS p55 with a core i7 or AMD old motherboard with Phenom II x6 or a new UEFI z81 motherboard, will it support Sapphire r9 Nano(legacy) and r9 Fury (UEFI)?

3) If a motherboard has UEFI BIOS(and has a pciE 16x) will it be able to run any graphics card which has UEFI support? same question goes for legacy.

4) Does this UEFI/Legacy support ability also depend on operating system? If so then let me know some details about windows 7/8/10 graphics card support.

5) r9 280x sapphire or many other radeon device has a dual BIOS switch, what does it do, does it have to do anything with UEFI/Legacy support? Or is only for overclocking? In which mode should I set it?

6) Does this UEFI/Legacy only important for GPU? or is it crucial to check this support when buying SSD or other components as well?