I actually use SeaFile for some clients (since OwnCloud doesn't do client side encryption at all), after Wuala shut down last year. I actually was disappointed by the security and features too (https://forum.seafile-server.org/t/encryption-the-pro-added-...), you cannot securely share sub trees/dirs like in Wuala (with their Cryptree).

Is there a better FOS alternative?

The Wuala service did had a lot's of deadlocks, either on server or the client side, and customer service was not done secure: please send over the log files (included file names) or the client storage block, etc...