I agree that the ST had a better software portfolio of commercial grade midi applications. There is no doubt about that.
Where I do not agree is the midi hardware aspect. If you argue about serial port speed on an Amiga, there are and were plenty of fast serial interfaces add-ons (that outperform the Atari St line) which are pretty cheap (less than $20), and even available as new today. BTW, most Amiga midi adapters have multiple ports.
BTW, you can easily emulate an Atari ST on an unexpanded Amiga and use its software, you canīt do the opposite on an ST.