Flatpack has the big advantage that you can a) run apps that couldn’t run the “normal” way (integrated with the system), because of lack of dependencies or b) have automatic updates of apps otherwise upgradable only via ppa or manually.
But its big disadvantage is the space occupied on your hard disk: indeed any app in Flatpack is a “world” in itself, regardless not only of the system, but also other apps in Flatpack itself.
Therefore, be careful to add Flatpack apps, unless you need really them (and them via Flatpack).