What are some alternatives?
Sampulator - Make (and record) beats on your keyboard
Splice Beat Maker - Make and share beats in your browser
Caustic - Caustic is a music creation tool inspired by rack-mount synthesizers / samplers rigs.
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
Magenta Studio - Make music & art using machine learning
G-Stomper Studio - G-Stomper Studio is a feature-packed, mobile music production tool.