Why is backtesting an EA useless?

Expert Advisors also come with some downsides. Traders love having the ability to back test to see how the EA did in the previous market conditions. Keep in mind that past performance isn’t necessarily indicative of future results. Market conditions DO change; if an EA performed well over a specific period of time, no one can guarantee that it will perform in exactly the same way in the future. We’ve seen plenty of EAs that make money and then fall apart at some point in the future.