Using Mint for some years now, there are two main pain points for me. Both do not stop me from using Mint as my daily operating system, but they reduce convenience.

Default package repositories contain software versions that are long outdated (e.g. tmux, claws mail, neovim, libreoffice). Although this can usually be fixed by custom ppa or manual installation it decreases the benefits of a default package repository and causes additional maintenance efforts.

Laptop hardware / driver issues:

• When using nvidia graphics driver, FN+Fx keys do not change display brightness (although brightness hud is shown). When using xorg driver instead, these work, but the input for unlocking my luks volume at boot freezes and I cannot enter the password.
• FN+Fx does not enable/disable touchpad. I was able to fix this with a custom script and keybinding.
• Keyboard lighting cannot be controlled by OpenRGB and some other tools I tried, because the specific keyboard is not supported (yet?).