I echo everything that Andy has said here. I have never had an issue with handoff between Unifi APs. Even when I was using Voip through Google Voice/Hangouts, my calls wouldn't drop even though I could see the signal dropping and rising as I walked from one AP to the other.

So no, a controller isn't needed. I didn't have one running for the two years I ran my Unifi system.

Of course, if I wanted a guest portal where I charged weekend visitors for internet access, then I'd definitely need the controller to be running...