Atari ST also booted from ROM. But it also expected a floppy disk to be in the drive, to check for auto boot programs, etc. So that slowed the boot. If there was no floppy, it would hang for a while waiting for one, even. Poor choice.
In RISC OS that was optional. There was a setting[1] in NVRAM which set whether or not to look for a boot device, and what that boot device was (floppy disc, hard disc, network).
I don't remember what happened if you configured it to look for extra boot files on a floppy disc, but the drive was empty. I think it would give up very quickly (1-2 seconds), as it was a normal way to load a program on the earlier BBC computers — insert the program disc, which would be bootable, and press the key combination (Shift+Break) to reset.
"Podules" (expansion cards) could also map extra modules into the OS from their own ROM, usually the required device drivers for the card.
