Someone else could (and has in other threads where manjaro came up) answer better than me for general reasons, but for reasons that personally affected me - version mismatches due to them holding back releases, driver issues (with an amd card), general app installation/updating issues.
Audio issues due to poor defaults, which as a beginner (at the time) user was difficult enough to diagnose I uninstalled plasma (twice) trying to fix (yes, that part is my fault for not understanding what pacman -Rcns actually does).
The installer is using a very incomplete timezone list that does not include any GMT -8 timezones at all (which isn't manjaro specific, but makes me leery of a dev's attention to detail when they use this list).
For the general comments I have seen others mention, they have accidentally ddos'd the AUR on more than one occasion, they let certs expire regularly, they hold back updates without actually doing anything to confirm the updates are stable when they do push the updates...
As for endeavouros devs being part of a discontinued project, I can't say anything that would bring back your trust as I am not part of that team, but they did do a write up about this on the endeavouros website.