Just as an example, A small project Qubes OS supports UEFI, but a lot of the UEFI implementations from different manufactures are broken or don’t follow the standards. Qubes OS doesn’t have the developer resources to fix issues with motherboards or laptops only used by a handful of users, so when all else fails the solution is to use legacy mode.
It would hurt some projects.
Just as an example, A small project Qubes OS supports UEFI, but a lot of the UEFI implementations from different manufactures are broken or don’t follow the standards. Qubes OS doesn’t have the developer resources to fix issues with motherboards or laptops only used by a handful of users, so when all else fails the solution is to use legacy mode.
Coreboot also uses legacy boot for some payloads.
Not really related to what Red Hat is doing with their OS.