Much of it is that it's a truly distributed system, so each computer you connect is untrusted - it doesn't fit well with use-cases of people migrating from Dropbox. It can be made to work, without all that much trouble, but it's based on a very different set of assumptions and it shows.
(I say this as a mostly very happy user of it, and I do recommend it. But you're not going to be successful in migrating your non-tech-enthusiast friends or family)
Examples: my likes on Twitter, highlights on the web (highlight something, right click "Import to Readwise", it downloads into Emacs via syncthing), calendar events, etc. I'm always collecting stuff that I can reference later without much effort after the initial setup. It just works.
It makes more sense if it’s combined with for example wireguard