Indicators are just an added source of information to help you understand various trends of the market and make a better investment decision. You cannot completely rely on them. Doing your own research is also important.
Having only one indicator in your trading strategy is a bad idea. THe thing is that there are no indicators which are coorect in 100% of cases. All the indicators give false signals which can be caused by many factors. So, if you want to make your trading strategy more successful, you'd better use several indicators which will give you better understanding of what is happening on the market during the given period.
The role of indicators shouldn't be overrated. They are only the tools which help a trader make decisions. The first thing you should pay attention to is the chart itself, and only after it you can you can turn to indicators which will confirm or reject your ideas. More than that, you'd better use several indicators at the same time in order to get a clear-cut idea of the current market situation.
In my opinion, indicators are reliable in forex trading. I base this opinion on my own personal experiences with indicators and the experiences of others that I know that use indicators. Indicators’ utility differs from person to person based on what they like and the kind of strategy they want to use.
There is no guarantee that all of the indicators are reliable and professional traders should understand this as well as rookies. There is nothing in trading which can give you a hundred percent result. Everything depends on the combination of factors. If we speak about fundamental analysis, then I can claim that if you're experience enough in it, you can predic the future price movement. If we speak about technical analysis, then there is also a probability that you will guess the future price direction, nevertheless it's not hundred percent. Be careful with it and never rely on your conclusions on hundred percent.
Of course, indicators are reliable. There are some good trading strategies based on indicators. However, one needs to learn and do enough back testing before using indicators. Time frame is an important factor as well. Some indicators work well on particular time frame, which needs be found out by traders.