Depends what you mean by “problem”. The biggest problem with traditional packages like debs and rpms is that compatibility sucks. They only reliably run on the distro and version they are designed for. Third party packages typically build on old dependencies and hope that backwards compatibility will allow them to run without issue on later distro versions.

Yes, it’s redundant to have have the same app packaged as flatpaks. Though I don’t think that redundancy is necessarily a bad thing. Flathub is not a profitable project and has up to this point relied on Gnome for funding. There’s work being done to spin it out to be it’s own thing and hopefully be supported by paid apps. But what if that fails and it shuts down? Or less dramatically, what if Flathub has a major outage?

One of the common complaints against snap is that there is only one store, controlled by Canonical. Flatpak is designed to support multiple stores. I don’t see why they can’t exist side by side. That’s exactly what I do. I have dozens of apps installed from each source.

And to address the claim of what if “each distro decides to make a flatpak repo according to their own philosophies?”. I guess that would depend on how many resources are being poured into supporting that. If flatpak continues to push for OCI support, then that would make it easier for distros to have their own remotes, if they desire. If not, they can just use an existing option. Whether that be Flathub or Fedora. Personally, I think Fedora Flatpaks are a good match for Debian and OpenSUSE’s policies, only real downside is that major Gnome app updates would be a month delayed, annoying Tumbleweed users.