Not a fan, mainly because it’s proprietary. Canonical hosts the only repository and you can’t change it. The file system stuff is secondary.

I do think Flatpak is alright, and I use a few Flatpak apps, but since I use Arch, I generally prefer to install things from the AUR.

As far as I understand it, never, unless someone makes an API proxy. They both use activitypub for federation but the client side API is totally different.