Snap is just up Canonical trying to build an AppStore that they control so they get a bit of every software sale on Linux. It's straight up evil. They neither support third party repositories nor is their AppStore server Open Source. It's build such that they retain all the control and only employs the minimum amount of Open Source to get away with it.

Flatpak is more tricky, I am not sure there is any company behind it actually controlling it directly. But it is very much build for KDE and Gnome apps. As a general Linux package manager it's completely useless, as it has no dependency management, the only thing you can depend of are the KDE and Gnome runtimes, there is no separation of individual libraries and such. Support for more than one binary per package didn't exist last time I checked, the support for command line in general is terrible and the whole thing is geared towards an Android'ish experience with simple monolithic apps you can click on. The fact that it runs on multiple distributions is great, but everything else about it is awful.

There is also Nix, this is by far the best package manager we currently have. Runs on all distros, completely reproducible builds, git repositories can themselves be treated as actual packages, everything is easily overridable and changable by the user and best of all it is all build on regular Unix tooling, i.e. just some symlinks and environment variables, very transparent and easy to understand, no weird container magic that hides what is going on.