I think the Amiga affinity for broadcast was that, iirc, it had either built-in or as an expansion, a genlock device.
Regarding ST for music, did you ever come across the Intelligent Music products? My roommate was the guy who wrote "M" and "Realtime", the latter especially being a really cool compositional tool. He did some stuff in that UI that was really innovative at the time.
The Amiga had the ability to sync the clock to an external video signal, and it also had a "color zero" signal present that would allow a genlock to know when to switch between the input video and the Amiga's video, so that you could superimpose Amiga graphics on top of video. As long as your input video was clean enough (my limited experience was SVHS without a TBC), the Amiga would remain stable.
Regarding ST for music, did you ever come across the Intelligent Music products? My roommate was the guy who wrote "M" and "Realtime", the latter especially being a really cool compositional tool. He did some stuff in that UI that was really innovative at the time.