What are some alternatives?
Sampulator - Make (and record) beats on your keyboard
SunVox - SunVox is a small, fast and powerful modular synthesizer with pattern based sequencer (tracker).
Splice Beat Maker - Make and share beats in your browser
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
Radium - Radium is a music editor with a new type of interface.
NanoStudio - NanoStudio is a recording studio for iOS, OS X and Windows.