You're right that I could get just as good results with other potentially faster algorithms but those algorithms don't afford me the ability (especially decision trees) to personalise.

The system is designed to be personalised. So for example if it's not doing well in recognising your squats for instance, you can immediately 'show' it what they look like. A neural network can immediately integrate that, and spit out great accuracy levels. Whilst i'd have to reconstruct a decision tree each time.