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.
The free tier is pretty good for many people. It’s mostly Photos that ever push people into actually paying for an iCloud account.