They are just indicators and you need a bunch of them showing signal signs to consider a trade. They all have some % of success and all combined makes for a high probability SIGNAL. This is still not confirmed that it will produce a positive trade. MACD and RSI are popular because they are very easy to understand. Problem is they are only oscillators and they can show you some change in tendencies but there is no way it will be a signal on its own and building strategies based on them is rather bad idea. You can use them as additional filters but not the main source of information.