They take the intuition out of trading. Buy when the system tells you to and sell when the system tells you to. The problem is that there are not a lot of good trading systems out there. However, some are created for certain institutions to take advantage of arbitrage opportunities, or tricky derivative strategies. They are not at all suitable for the average trader.
Traders tend to lose objectivity when using technical analysis indicators.
The trader is not able to remain objective and the subjectivity of using the indicator overwhelms him. I think it has to do with a compulsion many traders feel to have to trade. They then tend to subvert their indicators by trading when the signal is not entirely clear. Traders have a tendency to test their trading systems and technical analysis indicators on an insufficient amount of data. Analysts need to test trading systems and technical analysis indicators on a wide array of data in different types of trading markets.