I don’t remember anyone mentioning Snap being closed source, but it receives many complaints for interfering with the functioning of common programs, on top of slowing down the execution of programs installed through it and is now being forced on users. I haven’t touched any *buntu distro in years, but it always seemed half-baked from the comments I keep on reading about it.
Also yes, Flatpak is what I believe you could call a universal package manager. Package it once and it should run on any Linux distro since it takes most things out of the equation, save for the kernel and drivers. And yes, it mostly is used to distribute desktop applications. It’s ideal for safely running random applications or older programs that wouldn’t run through a modern runtime.