Midi is the one area that the ST does beat the Amiga in. It isn't just the hardware, it is the amount of software available for it. I have a Mega 4 ST with a Unitor N dongle running Notator. Nothing even came close for the Amiga for Midi sequencing. For timing, it is better than anything for the PC, Mac and Amiga. There was Cubase for the ST as well and a number of other sequencers as well as a plethora of synth editors, patch librarians and other Midi utilities. These were all developed over a period of years for an active community of users just like the Video Toaster and Flyer were for the Amiga.
The Amiga Midi interfaces use the RS-232 serial port which is pretty limited. It works fine for just one Midi port but it is not fast enough for multi port applications. My Unitor N has 2 additional fast ports and there was another dongle that attached to the St's serial port that gave it even more Midi ports but the Notator manual warns against using them for devices for which the timing is important.