`pacpak -Syu` would therefore always install exactly the same version of the software as available with regular pacman.
-Syu with pacman means refresh databases and install all available updates. does this mean pacpak execute this logic on all installed containers as pacman executes on all installed packages? especially the refresh part makes no sense to me at this point. what external database is there to refresh? an install command would likely look like -S like in pacman? whats the base for installation? PKGBUILDs (from AUR/ABS), official repos, some new platform containing build recipes for pacpak?
it can be used to create containers from existing Arch packages.
sounds like working from /var/cache/pacman.
i generally like the idea of isolating and running untrusted software, as it allows one to implement sth like an applicationwise firewall (as done in android. wether this reduces or increases attack surface heavily depends on the implementation and its possibilities, but running malicious software will never be safe. this only adds another layer of control to the os.