RS232 is pretty limited and not much different from the Amiga to ST or PC. I don't know of any Zorro card or parallel port MIDI port interfaces for the Amiga. True you can add more serial ports by getting serial port Zorro cards. I belive the GVP IO extender board was even set up with Midi in mind. Both Notator and Cubase had port expanders that used the St's cartridge port. Even if you get the hardware to perform equal to the ST, the software for the Amiga is lacking.
The St's weakness in the OS department was actually an advantage for midi programing because the programers weren't restricted by trying to conform to strict OS guidlines and used a lot of assembly hacks. Notator totally rearranges the ST's memory and is incompatible with a lot of other St software. You basically have to dedicate your ST to it if you want to use it.
Way back when--like 23 years ago, I had access to a music studio that used an Amiga 500 for sequencing. I found it it pretty lame but the graphics were great which led me to buy my A2500.