Unless you encountered a BIOS that actually followed the spec instead of blindly loaded a sector and hoped for the best, and learnt that your typical linux install programs made malformed MBR that does not mark the drive as bootable so BIOS actually should skip it when booting.

Or when grub suddenly stops working on a dual boot system, and after you fix it another part on the non linux system fails, and if you fix that grub fails (and fails in all cases hard enough you need to boot from something else). After some time you either ended up dropping either grub or $some_important_software - poor you if the latter is something you make money to sustain yourself. All because, surprise, you're not supposed to write bootloader code into cylinder 0 outside of first sector, and various signature systems used in licensing use the no-mans-land of cylinder 0 to scribble signatures.

BIOS offered even less debug tools than (U)EFI, so dunno what you're talking about. UEFI shell is documented, if not as well as it could be, and even has built-in help. That you can, worst case, boot it (or any other tools) from simple fat32 formatted pendrive that can be created with a file manager instead of rawdogging the hard disk is just a bonus.

The cases with Linux entries being ignored were plain bugs by specific manufacturers, bugs that also hit windows (because the vendors in question worried that them not checking properly would backfire, and instead of checking the firmware properly hardcoded few tested bootvar names).

And the ignored entries could be resolved by using standard fallback bootloader paths, or renaming linux entries to same as ones that work.