SFML has been used in multiple apps on Android and I believe there were some in development for iOS.
The API is pretty stable and it's certainly working to some degree. The problem we have, is that we don't have enough testers who then also report back issues and also who help solve them. Since the Android universe is so vast, it's impossible to test everything and sometimes error only occur on a specific device, so we do really rely on people helping out finding a fix.
Additionally we're still looking for an active maintainer for iOS.