I tried dual-booting Manjaro from my Ubuntu install, since VMs were slow on my machine at the time and I wanted to give Manjaro a try.
Manjaro wouldn’t boot (X11 sessions crashes on boot), and then when I returned to Ubuntu, I got dropped straight to the GRUB rescue shell because I had shrunk the partition from the Manjaro installer, and it had fucked up the Ubuntu install :/ so instead of two OSes I had none
For Flatpak apps, along with Warehouse, Flatseal allows you to view and edit permissions for each app, which is not only useful but sometimes mandatory when an app has misconfigured permissions