AppImage is a nice idea, and avoids some of the performance overheads from containerised systems, but lacks a reasonable self update mechanism, lacks code signing and the desktop integration (having icons show up in the start menu) is poorly implemented.

Snap is essentially a Canonical-proprietary apt replacement with some very serious drawbacks around performance and desktop integration (themes).

Flatpak has some drawbacks but it largely achieves it's design goals, and actually provides some advantages over installing things via the system package manager.