KDE Flatpak apps are in kdeapps:

So, you could use remote-ls command to see what's available - far from easy to use flathub. I am using only Angelfish. There is a hope that Spectral (Matrix client) will get ported to Kirigami and will be usable on mobile. So far its not.

As for other apps, I haven't tried much. Many don't work, as far as I remember. For me, browser is good enough reason to use it

PS: Pure Maps and OSM Scout Server do work in Flatpak, except location is not known (Pure Maps). But they work better natively ...