If I were to take a guess, it would be that some people think the UEFI spec is over-complicated and tries to do too much things, and that the firmware-level is

If I were to take a guess, it would be that some people think the UEFI spec is over-complicated and tries to do too much things, and that the firmware-level is not where all these things should be done. It would have been fully possible to create something better than BIOS without (effectively) creating a new mini MS-DOS inside the machine firmware. The buggy UEFI firmwares some people has had to suffer also lends hand to that argument: A simpler specification would probably have caused people fewer issues. Sometimes less is more. I suspect UEFI would have been more warmly received had it taken a lower aim. It probably didn't help either that it was initially (wrongfully?) associated with Microsoft, and a hostile anti-Linux takeover of the X86 platform at firmware level.