Kubuntu chose to not ship Plasma 6 because it didnt match their release point. While in general a good decision, stable releases are the flaw really.
So a semi-rolling Kubuntu yes, but LTS Kubuntu before Plasma 6? Wouldnt recommend.
Ubuntu is also still full in on Snaps. Flatpak comes from a PPA I think. Their store is snaps-first.
While this is not a bad concept, Flatpaks are better for many reasons. They also may be worse for some others, mainly because Snaps just use Apparmor for sandboxing, like the rest of the system, so you can make easy exceptions.