I guess the biggest issue that’s been mentioned on the forums before is microG - here’s a discussion from the DivestOS dev on why there’s issues to consider with microG vs GOS’ sandboxed google play.
Also I’m not entirely sure if they still use it (the calyx website says they do at least but if they no longer do this point is moot) but the Bromite browser hasn’t had an update since December last year (as per the bromite github repo) which is Not Good At All considering it’s what’d be used for the webview. Having an outdated browser is really bad and I hope that the site is just outdated and they’ve actually moved to e.g., Cromite.
Providing the provisions to “easily” install private applications is kinda moot when there are issues with that - I don’t have a source for this beyond remembering a discussion from the GOS matrix rooms, but if I’m remembering that discussion correctly, the included apps with Calyx are sometimes outdated. If you have to update the apps basically immediately after installing, then you might as well go to the up to date source anyway?