Here are some of my thoughts on the flaws of traders when using TA. But for the record, let me say that I believe that TA is very effective in assessing crowd behavior and works well most of the time when used correctly. It isn't perfect and trading is a probabilities game. I am a discretionary trader. I have tried to come up with a trading system, because it would make trading a lot easier IMO, but it is too difficult to program my thought process for taking a trade.
I believe that many traders do not have an edge with TA because they rely on indicators alone, which always lags the market, and on top of that, they wait for price to turn to trigger a trade (i.e., buy the offer when going long and sell the bid when going short). By this time, the trader would have lost most, if not all, of their edge because of commissions and slippage. Then there are many traders that do not have discipline and let their losses get too large.
Seondly, I think all traders should learn to read the price action and then use indicators just as an aid to see what is already in the chart. If you learn to read price action, you will always be faster than most who rely on indicators alone. For example, one of the few indicators I use is a modified MACD to indicate momentum. In general, I do not need this indicator, because I can see that if the last up swing is greater than the last up and down swing I can tell you that momentum is increasing and that price highs are yet to come. Secondly, I can gauge trend by looking at price as well. The MACD will show me momentum and trend but I will see it first on the chart. Another example, is that a long consolidation pattern will usually lead to a powerful move in the direction of the breakout, why does any trader need an indicator to see this? I do use 3 indicators in all of my charts and they are the same regardless of timeframe. Again, I use indicators as an aid, but I also know when not to use these indicator. Each indicator tells me something different about the market, so I do not have more than one momentum/oscillator on my chart, for example. MACD, CCI, Stochastics, RSI, etc. they all tell you the same thing.
The other problem that I've noticed with traders is that they do not know when to use strategies for trading in a trading range or in a trending market. I've read many posts on ET where the trader says that they made good money on 19 out of 20 days and on the 20th day they give most or all of it back. Key is to learn when a trend day is unfolding and change your strategy or stand aside, because using a countertrend strategy in a trend day is detrimental to your bottom line!
Sorry for the long post.