This holds up for simple file hosting, but the entire suite of Apple apps supports collaboration, many with live editing features. Slow sync times would put a damper on that.
I don’t use logic, but yeah the whole iWork suite, Freeform, Reminders, Notes. I think HomeKit data is also synced, and given how much data corruption happens in HomeKit as it is, I would dread any other complication.
There’s also the transparent encryption that happens where even Apple can’t get your stuff.
I mean I’d be a fan if Apple published a requirement spec for the iCloud api and opened it up to third parties, but it will be a lot to unpack and implement.