The St's great virtue was its simplicity. It was a much simpler beast than the Amiga. The OS was entirely in ROM. That included MIDI interupt handlers and bios functions which made writing MIDI software for it very easy.
MROS was just one of many 3rd party overhauls of the entire TOS Gem OS. MROS was specialized for the Steinberg software and MIDI. There was also Neodesk, MultiTOS and quite a few others. The Atari OS was so lacking and incomplete that it inspired all kinds of hacks. TOS got a little better with the STE and TT versions but it still was extremely simple compared to Amiga OS 2.1 and 3.1.
Some of the Atari MIDI software had Amiga versions. Dr T's comes to mind. Steinberg's Pro 24 as well. There is nothing like Notator or Cubase for the Amiga but there is still quite a bit of MIDI software. For doing a fairly simple setup, I think the Amiga would be OK but I don't think it would would in a complicated studio setup with multiple synths and controllers and outboard MIDI devices like mergers and MIDI patchbays. The Mac of that era could but not the Amiga. It wasn't that the Amiga was incapable of that on a hardware level, it was just that that capability was never developed.
Interesting side note. A lot of the keyboard synthesizers, drum machines and other gear from the 80s used the MC68000, the same processor that the Amiga, ST and Mac used. Almost all of the later Sequential Circuits products were 68000 based and I think a lot of the EMU and AKAI gear was too. A lot of the better stuff had SCSI ports which was a really expensive option at the time.