I think flatpak is probably the right choice for a phone. Assuming equal developer attention on both flatpak and deb repos, there's nothing inherently worse about flatpak.
The nice thing about flatpak for a phone is that it could in the future offer something like the Android/iOS permissions interface.
Yeah, maybe I am a bit too negative about the format itself. My problem with the Flatpak only approach was that it was kinda hard to get it to run at all and it didn't work particularly well, so I ended up with putting some work into it while ending up with an unsolved problem (stable Linux desktop SIP client).
So maybe this is not an inherent problem of this technology and will work better in the future.
The nice thing about flatpak for a phone is that it could in the future offer something like the Android/iOS permissions interface.