> UEFI's just a more modular/uniform sort of BIOS.
I don't know. The BIOS usually seems to work, whereas UEFI usually has so many bugs (in my experience) that it is hard to get to work. So if you find bugs without looking for them, that would indicate that you can find even more if you are looking for them, most likely with security implications.
Some people say that UEFI is too complex - and the evidence seems to support that notion. All a boot loader has to do is to load a binary from disk into RAM and execute it. BIOS got that right - but unfortunately the boot sector of 512 bytes is way too small for modern software. Let the boot loader say how long it is, and load everything into RAM. Any decent kernel can deal with the rest, using hardware discovery etc.