About a decade ago, this worked great. All I had to do was create a symlink within my Dropbox and the directories were backed up.
Then a few years ago, Dropbox stopped supporting symlinks because they “caused problems”. It’s unclear what problems exactly because I’ve never actually heard of anyone who experienced any issues. Nonetheless, the solution was to move the real directories into Dropbox and put a symlink in their original location. This _did_ cause problems, including needing to disable SIP, and breaking navigation in Finder which treats symlinks like shortcuts.
Finally, the latest update completely broke backups for me. Admittedly this was not Dropbox’s fault; they were forced by Apple to use a new API which doesn’t allow the Photos library to be backed up.
I moved my backups to Backblaze B2 and cancelled my Dropbox subscription. Now I use rsync to back up my machine, and utilise Dropbox’s free tier for the small amount of cloud storage I actually need.
Oh, and for the amount of data I back up, B2 is almost an order of magnitude cheaper.
If the reason for that was really written with such vagueness, it's almost certain that there was some sort of vulnerability / Security issue. The most vague, the most serious the issue. If it was just "they cause problems" then someone somehow figured out how to use symlinks to get access to someone else's data.
It depends. When I was working on security software, any problems that impacted security were explained to customers in great detail.
Problems that were merely embarrassing, though, were always described in the vaguest possible terms.
- some users want symlinks to be synced as symlinks (and they might not know it, old Apple file formats have internal symlinks), so their files got corrupted if they were accessed on another computer - some users want the sync client to follow the symlink and copy all the files at the destination, which requires the client to listen to filesystem events for potentially the entire filesystem, which is no longer permitted in many cases
so, not exactly a vulnerability, but a mix of security and product issues.