This is what's so great about Linux, you can use whatever the hell you want.
Flatpaks provide some cool security functionalities like revoking network access to a specific application. Maybe you care about this, maybe you don't.
My personal policy is to always install from the repos. Occasionally something is only available in flathub, which is fine for me. I really understand how hard is maintaining something for every single package manager and diatributions and totally respect the devs using a format that just works everywhere. If I were to release a new Linux app, I would totally use flatpak.