Yeah as they said it’s complicated, but in an unintuitive way more sandbox of apps can lead to apps being less effective at sandboxing themselves. Which, like you said, can be good bad or neutral depending on your threat model.
Personally I am leaning towards not using browser in Flatpaks since I trust the browser to sandbox itself. Not the position I started from initially where I would have assumed more sandboxing is a uniformly good thing.
So can you just swap compositors for Gnome or is Gnome reliant on their own compositor (Mutter)?