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)

It's not a bad shout for beginners by any stretch, but it has a massively overdone reputation for beginner-friendliness that is not really deserved

Cinnamon, for one. Yes, it looks kind of like Windows. But the similarity is surface deep, and it's also pretty janky- by far the biggest resource hog of all the main DEs, lots of weird snagging bugs and stability issues. I've always found it very unsatisfying.

I personally use MATE quite a lot and I enjoy it, but I wouldn't really be recommending that to Windows users either; it's pretty old school at this point.

Keep recommending Mint to people by all means, though. If you like it and it's what you use, that's still a great recommendation. There is fundamentally no reason why beginners shouldn't use it as their first distro.