AUR appears to be quite popular but sometimes software is flagged out of date or it is dormant because the maintainer has no time any longer and new maintainers may come and go. Basically anyone can upload to AUR and there is no curation. Of course you can look at entries in the AUR and see when software was submitted for the first time and how many people have upvoted it but even that can give a false sense of security. The disclaimer at the AUR is clear : DISCLAIMER: AUR packages are user produced content. Any use of the provided files is at your own risk.

Flatpak has a drawback of pulling in more software that an application depends on (For example GNOME or KDE libraries) and someone has made a website https://flatkill.org of their bad experiences with Flatpak but that is from 2020. In the mean time there is Fedora Atomic desktop flavors depending on using Flatpak.