Isn't it just a thin wrapper around `flatpak permissions`?
While obviously a handy GUI, I've never been able to overcome my trust issues with allowing a random package from GitHub to govern permissions for my Flatpaks.
Ironically, I'd probably trust it a lot more if it came as a Debian package. There's a circular issue here where the people who are security conscious enough here to want something like Flatseal might also be too security conscious to install something like Flatseal.
[0] https://flathub.org/apps/com.github.tchx84.Flatseal