Installed on new Mint 21.2 install without issue. Flatpak is convenient as it updates automatically, although generally a couple days behind the AppImage on nextcloud.org. Note must be manually added to "Startup Applications" after first install.
When I realized that the version in the software center was the actual version of the app, I installed it instead of the AppImage. Seems to work fine. I had to add the "Nextcloud Client" to the "start programmes" manually, though. Seems to work fine now! Therefore, I'll rate it 5 stars. I am using Linux Mint 21.1 (Vera) Cinnamon Edition.
I do like the app. But I always have issues with auto-start the app during startup. It never works. Even if I check the checkbox in the settings, to start during the boot.