UEFI isn't really a "problem", just an unnecessary piece of bloatware (in my opinion). All you really need is basic hardware initialization and some kind of bootloader (in libreboot we use GRUB. on non-libreboot systems, uboot etc are common). The issue is much lower level than UEFI. Nowadays, it's common for systems to not allow modified firmware at all, making libreboot impossible. It goes beyond just UEFI, which is a limited application, and by no means universal. Most of the features on modern Intel systems preventing Libreboot (ME/CSE, intel boot guard, etc) have nothing to do with UEFI