Flatpaks dont touch the system, which makes them a perfect addition to a Debian Base. Tbh I think Windows is the best example that this works, stable, boring base, and no software is stable for no reason. They outsource the work and the software even installs and updates in random ways, but it is always up to date which never breaks the system.
But to be fair I am not a Debian user. I would consider it when being an admin for many clients that want a stable system. But I would install all apps from Flathub then, to have them up to date and not years old.