IBM PS/2 Style BIOS booting required to write a boot sector in real mode assembly, and a kernel that needs to be able to handle the ISA memory hole at 15 MB. The bootsector is written around BIOS routines for disk i/o. The boot sector is not specific to any file system. Linux used to be bootable from a floppy disk that only contained the raw linux kernel without and file system.
Writing a boot sector is not a difficult effort. In fact, its an exercise frequently used at universities to teach students about bare metal programming.
With EFI, you are now required to use the GPT partitioning schema. While the spec specifies big endianess, GPT uses UUID's with mixed endianess (Chen is right that it is technically all little endianess, but this is still a problem for anyone who is not using Microsoft's struct format). So, for each textual UUID's there are two binary representations. There needs to be tooling support to toggle between the binary preresentations. PXE has the same problem.
There is also only one filesystem type universally supported by EFI, and that is the FAT file system, which was originally written in 1977 for floppy disks, 4 years before the first machines booted using the PC BIOS. FAT originally supported only 8.3 filenames. Microsoft came up with the VFAT long file names which use additional directory entries to encode the longer file names in UCS-2, a long obsolete unicode encoding. So you need support for VFAT and UCS-2, too.
Also, EFI binaries are Windows Portable Executables, which is an extension of the MZ executable format introduced by MS-DOS. Every EFI binary still has the compatibility stub for MS-DOS, even if it doesn't contain any useful code.
So its not difficult to see that EFI itself comes with a big pack of legacy, which is no less problematic than it is with BIOS booting. Take note that this baggage exclusively comes from Microsoft, which used to be one of the largest enemies of free software and probably still is.
Yes, EFI now allows for secure boot, but this is frequently used to lock users out from installing Linux on the Laptops, and is only as trustworthy as your firmware, which now supports modules being installed, potentially from malicious entities.
One major annoyance is that BIOS is decried as "legacy" boot method (legacy = old = bad for people without critical thinking) by influential people, suggesting that EFI is better in any way. And then decrying the people critical of this as "reactionaries", which is a dishonest approach.