Forced Snaps is a big one. If you're not familiar, Snap is Canonical's proprietary alternative to Appimage and Flatpak. While the Snap Store is open source and can be forked or modified as needed, the backend is completely closed source, which has vexed many members of the Open Source community.

While the distribution itself is currently pretty solid, they've made questionable decisions in the past like including an amazon search function in their fork of gnome (Unity). Snap can be removed by a skilled user or someone well versed in search-fu, but their choice to have it installed by default, the be the default for package management, and to inject snaps in place of deb packages when installed via Apt, are all big red-flags given that nobody can see what is in those snaps til they're installed except for canonical.

I don't know about anyone else, but, I went from Kubuntu to Debian/KDE because I don't like seeing all the Snap-fake hard drives in lsblk.