Yes and no. For example, we have trained over 800 traders using the MAX Trading System. We know it is a highly profitable approach to trading the markets. However, it takes much more than great trading methods to make a great trader. The psychological elements of self-discipline, greed, fear, hope, confidence, etc are at least as important as your trading methods. So, you may indeed have a profitable strategy, and yet be a losing trader.
This is a big part of why we ask our students to achieve the 3 consecutive weeks of profitable demo trading using the money simulation approach. You absolutely must have some way of feeling the stresses that are generated when you trade "real money." As KalixMOR said, the markets are hard. It is a very unfriendly environment, and without a good money simulation to experience the pain of losing, demo trading is of limited value. The issue of the difference in demo and live fills does not affect our MAX traders, but the psychological issues cannot be avoided. If you walk into the live markets unprepared, you will exit the markets wishing you had invested more time and money in preparing yourself for the battle...only now you may not have any money!