Flathub sandboxes applications to prevent them from infecting the system if app gets hacked, and lets you control permissions for each app, like microphone, camera etc.
But they are still work in progress and easily circumvented, thus I called them only minor improvements.
The only real issue with Ubuntu is it uses Snap app store instead of Flathub app store.
Flathub has more apps, is faster, and has some minor security improvements. Linux Mint uses Flathub.
HOWEVER, you can add Flathub as additional source in Ubuntu app store app, and use Flathub exclusively.
In that case, no real difference between Ubuntu and Linux Mint other than UI.
They have had their Store for over 10 years now. I envy you if you haven’t had to deal with Windows for so long. It’s not even user-friendly anymore, it just hates it’s users and constantly tries to scam you with ads and pop-ups