There was a point where I had most of my work and ideas in Devonthink. Then I noticed that a few important files had somehow vanished. I don’t know for sure if this was due to the software, but it unnerved me, and I moved to Obsidian.
I used DT primarily as a knowledgebase/notes app; I never found its smart features to be particularly useful. Nowadays, I’d definitely recommend Notion over DT.
I lost four to five years worth of data, the database just vanished. The backups I had were useless (some sort of systemic corruption?). Having been badly burnt, I can’t go back, but I’ve yet to find a tools that matches my workflow as well as DT does. Bummer.
I had something similar happen in Logseq when I stored my main directory in Sync, which is similar to Dropbox. After moving my main directory out of Sync, everything became stable again. Perhaps Dropbox was the root cause of the problem.
The problem could be the way you used the directory. I don't know Sync specifically, but DEVONtech don't recommend storing the databases in directories which will be synced. Rather you are supposed to set up the system to use whatever sync API is available, which presumably makes sure files are internally self-consistent before syncing. It's a while since I set it up, but that's how I did it with WebDAV to my NextCloud server.
I used DT primarily as a knowledgebase/notes app; I never found its smart features to be particularly useful. Nowadays, I’d definitely recommend Notion over DT.