I have used Dropbox and have setup Syncthing for myself and my opinion is that:

Sync thing is relatively easy for the average HN user.

Dropbox is relatively easy for the average non-HN user.

Neither are hard, but Syncthing requires some background understanding of computers

But Dropbox business model requires them to use accounts -- as far as I know you can't set up Dropbox on a shared device without signing into an account. This makes it harder than necessary to set up Dropbox in some scenarios.

Sure, they make it as easy as possible to get started, but "easy to use" for first time users does not mean "easy to use" for people who are willing to spend a few hours to read the docs.