Tried running #VSCodium in #Flatpak. It did not go well.
An IDE that can't access /usr/bin—wherein one finds compilers, version control tools, and so forth—is a rather useless IDE.
Technically these are still accessible at /run/host/usr/bin, but extensions will not look there, and many programs (like hg) will not run from there.
Such things can be run successfully using host-spawn, but again, extensions don't know that.
So yeah. Fail.