Initially I didn't like Fedora Flatpaks, but they have grown on me. What I really like about them is how they are built directly from Fedora RPMs.
Unlike Flathub, they don't pull and build dependencies directly off the internet (which may not get timely updates). Instead those dependencies come from Fedora's repos.
The way Fedora Flatpaks get built are also much more consistent since they just use Fedora's infrastructure. Meanwhile flatpaks on Flathub may be repackaged Appimages, snaps, tar.gzs, built from source, etc.
Though there is the obvious downside of Fedora Flatpaks which is missing media codecs. So if an app needs codecs, I just end up using Flathub versions. And Flathub for whatever isn't in the Fedora Flatpaks repo.
There is a second runtime for KDE stuff.
Ok good to know. Still possibly smaller and just an extension, not a whole duplicated runtime