A few off the top of my head:

• Every time I try it I have installation issues, across a wide variety of hardware. (Newbies have also reported to me that "Linux can't even install" after trying Mint - when I sit them down with a Kubuntu install on the same machine it tends to go flawlessly)
• Cinnamon seems to have stability issues (this is one of the more common things I've had now ie friends complain about and ask for help with)
• the blocking of snapd in the repos and the way it's done can be pretty confusing to newbies when they click a "get it on the snap store" button and things just fall apart. (I also think their blocking of snapd itself is fairly user hostile, but the fact that the UX around it is so bad is also a problem)
• On the subject of blocking packages in the repos - their own packages seem to have file conflicts with the Ubuntu repos they use but don't put the relevant "Conflicts" lines in their deb metadata, which I've seen cause conflicts for newbies that break apt. (KDE Neon does a much better job of taking care of this IMO, but I certainly don't view it as a beginner friendly distro either)
• The lack of a Plasma version is a major downside to me. (Random aside: I once had a newbie ask me how she could get the pretty version of Linux I had because hers was so ugly - she was running stock Mint and I was on Fedora's KDE spin)