The only downside I see right now is that password protected collaboration in WorkFlowy requires an upgrade. You can collaborate via a secret link though. Also, the free account has limited items but 250 seems pretty generous anyway.
If you want to check it out, here's an invite link that gets you 2x more free space: https://workflowy.com/invite/28e6274f.emlx
Let me know what you think and if you have any interesting use cases that might be worth considering!