Directory hard links are unusual and Apple only implemented it for Time Machine. It was never really well supported on macOS to begin with. Apple removed it from APFS.
It works, and I used to have a script to rsync certain directories into the Dropbox folder as hardlinks (I can exclude shit like node_modules that way). Not directory hardlinks, but rsync’s --link-dest, so that each individual file is a hardlink.
Anyway, I stopped using Dropbox long ago.
Edit: Parent is talking about directory hardlinks, so no.
For example let’s say you want to sync all of ~/Movies, could you hardlink ~/Movies into ~/Library/CloudStorage/Dropbox/Movies and it would work?
I don’t use Dropbox so I don’t know, but maybe someone knows?