

a lot of apps on the flathub website say “Unverified”
Those are usually either wrappers for proprietary stuff, for example the Chrome flatpak is unverified because it’s not from Google themselves but rather somebody grabbing the official deb/rpm and rebuilding it into a flatpak (this is also how a lot of e.g. AUR packages on Arch work, basically), or open source stuff for which the dev/packager simply didn’t care enough to do the verification stuff that Flathub wants you to do (doesn’t actually seem that hard, but one might simply not have been aware of it or something).
Don’t recall people particularly complaining about the unverified badges before Mint started hiding unverified flatpaks by default, though; suddenly after that “everybody” started noticing them.
I don’t really run much of any unverified ones myself anyway, though tbf the unverified proprietary wrappers on Flathub are at least somewhat more trustworthy than the AUR equivalents (at least it doesn’t get to run stuff as root during installation, like Arch packages (or any distro packages really) do), though in both cases you are giving them access to your $HOME so that’s something to be always considered.
I’m not really sure why Fedora Flatpaks still exists… I mean yes it sounds good as an idea (distro gets to ship sandboxed apps alongside conventional packages) but there’s still the upstream devs vs downstream packaging conflicts, and for new users it’s annoying at times because… reasons (the package you thought was coming from Flathub was actually pulled from the Fedora repo because it’s in there too, etc.), seems like effort duplication on top of the existing effort duplication that was/is downstream packaging but still.
Some distros do have their own flatpak repos as well but smaller than what Fedora is doing, https://appcenter.elementary.io/ for example (but a substantial of that stuff is primarily only available from there, though you can build it yourself), though again I’m not sure much of any other distros would want to implicate themselves with that because… all the reasons.
Flathub is a separate chain of packaging from the distro itself so there are legitimate reasons to avoid it if one is heavily paranoid though.