And they are still, in my experience, slow to load, a cumbersome addition to the update process, and often un-necessary.
Don't get me wrong, if you're in a tight spot and can't make two significant software packages work in a distribution due to conflicting library version requirements... some kind of lightweight container solution is attractive, expedient, and better than just not supporting one of the packages. But, my impression is that a lot of stuff has been moved into flatpak / snap / etc. just because they can. I don't think it's the best, or even preferred, way to maintain software - for the desktop environment.
(Returns to checking on his Docker containers full of server apps on the R-Pi farm...)