> It is, as far as I know, the only cross-distro package format that is designed to be cross-platform (desktop, server, IoT) and with built-in sandboxing. Moreover, it is feasible to base a distro off of snap alone (Ubuntu Core, designed for IoT).

It is not really cross-distro though, since different from Flatpak, Snap assumes many features to be enabled for sandboxing to work (like AppArmor [1]): https://wiki.archlinux.org/title/Snap#Installation. Flatpak only assumes, AFAIK, user namespaces support on Kernel.

[1]: This can be really problematic if your distro is using other MAC like SELinux, since while they're really not designed to run together.