Personal main-complaint about Snaps is that they ship Firefox by default with it and some things in it are just broken:
“Save Image As…” in the right-click menu would just fail to open the file dialog and therefore do nothing.
It doesn’t use ~/Downloads/ for downloads, but rather some complex folder underneath ~/snap/. You can get to that folder from Firefox’s download list, I believe, but navigating there via file manager is tricky.
As for Kubuntu, it’s far from the greatest showing of KDE. They frequently have oddball KDE versions, e.g. not quite shipping the KDE LTS version in Ubuntu LTS, because releases didn’t line up, but also just in general weird instabilities and crashes which don’t happen on my openSUSE laptop (my workplace issues Ubuntu laptops).
Having said that, we gave some of our Linux newbie colleagues GNOME and they always seem to struggle more with it than the colleagues with KDE, because usability in GNOME is just whack.
Things like not being able to type a file path into the file manager (unless you know the magic shortcut Ctrl+L), or the file-open dialog highlighting the name field, but when you type into it, it starts searching files instead.
But also just the whole thing not behaving like Windows. I’ll be the last to praise Windows’ usability, but it is what many people know.