I was using DropBox for a while.
Putting project in DropBox folder,
Syncing,
Turning DropBox off,
Working on the project,
Close UE4,
Start DropBox to sync the changes,
Repeat.
This worked fine since the project was shared only between 2 people and each of us knew what the other will be working on. If DropBox is running in the background and you launch the project it will corrupt the files.
But absolutely unusable for a team. Perforce is doing well for our team.