You can limit the file system permissions of the app, like giving only access to downloads, so that if/when there’s a sandbox leak you’re fine. You can also disable various things, like webcam or mic, this way.
In addition, you can get perpetual updates to the latest version of your browser even on old, stable distros like Debian.
Running a new browser on an old distro would be a strong reason for me (if I somehow couldn't update the distro - but I can and I do.)
Regarding security, the added work and complication outweighs the added security for me. I can't really disagree with having a different preference. More security on this wild internet is better, right?
IMO it's not much added work. In KDE you can navigate to settings and edit flatpak permissions, and flatpaks are available to download via discover. I haven't noticed any weirdness for firefox or chrome.
