Mechanical systems do not work.
Let me rephrase that. Backtested over several years of data, mechanical systems do not show a sufficiently consistent profit and an acceptably low drawdown for the small trader to trade consistently, comfortably, effectively and economically.
To expand on that further, the results of a coded mechanical system might well show periods of excellent profits but, virtually without exception, they will also have draw-downs that most small traders would not be able to tolerate. In addition, most systems tested over several years invariably have considerable periods of losses, usually running over months if not years, plus long runs of losing trades.
If you start trading a mechanical system at the wrong time...