Sure, some commercial robots are just out and out scams. For instance, an unscrupulous vendor might realise that his EA would have blown up an account on a certain day, so he will hardcode any bad dates into the EA to prevent them from taking trades in the strategy tester on those dates.
Unsuspecting buyers will see an equity curve produced by the tester showing a steady rise because it won't have taken any of the fatal trades. Anybody caught by a scam like this risks not only losing the purchase price of the EA, but also the contents of their account when the EA inevitably blows it up at some stage in the future!
In my view, this is just downright deception and such products rightfully need to be exposed for what they are!
Fortunately such instances tend to be few and far between, although there becomes a tendency to tar every EA vendor with the same brush. This is unfair because there are some good guys out there.
The worst enemy facing EA purchasers in my opinion though, happens to be the purchaser himself. I believe that most EA purchasers tend to have too high a level of expectation and wrongly think that they'll be able to make their million from an EA within a few weeks.
Life isn't like that though. Most novice traders don't understand that losses can happen regularly. They don't understand that losing spells may last for several weeks or months on end. And, above all, they don't fully appreciate the emotional turmoil that people face when losses inevitably happen.
Realistically, a professional trader should be looking to double his money each year. To do this, he will invest wisely and he will not risk too much. His primary aim will be to ensure that he still has sufficient funds to return to the market. Let's face it, most banks are paying savers peanuts for interest at the moment, so anybody who doubles up in a year will have easily beaten The Street.
The same should be said of expert advisors and this is probably because they tend to be marketed towards inexperienced traders. Most are deemed to be failures because the user has an unreasonable level of expectation. Users will frequently trade them with too high a level of risk, and won't do the necessary tests before putting them on a live account.
Tests are very important because they show whether the robot is likely to endure lean spells. They will show the true consequences of a few losing trades in succession. They will also show the right level of risk to apply when setting the robot up. They will even show whether the robot is suited to a particular broker.
Forewarned is forearmed, as they say, and if all robots are tested properly from the outset, it should be seen that many don't really fail at all. All that is often needed to make them work profitably over a sustained period is a little patience.