Ubiquiti recommends using mongodb 4 or later with Unifi.

Unifi in pkgsrc can use either mongodb4 or mongodb6

Recommended process of switching between them:
- Login to unifi, and download a full backup
- Shut down unifi
- Install new unifi version
- Rename away the old unifi/data directory (this also provides
  a rollback option if you need to revert) & create new empty data
  directory
- Start up unifi
- If you have customised data/system.properties for unifi.http.port
  and similar, shut down unifi (now that a new data/system.properties
  is present), merge the changes across in an editor then start unifi
- Go to the web interface and select upload backup