The biggest advantage of robots is their consistency. Once activated, they stick to their trading strategy and money management no matter what. Nothing can ever cause them to rush into a decision or invest out of frustration.
Especially traders lacking the mental endurance to become successful traders can greatly benefit from letting a computer take over their decision making process.
Robots are computer programs that can easily contain many thousand lines of code, many of them have over 50,000 or 100,000 lines of code. That is one of the biggest drawbacks to robots: Most trader lack the ability to create them on their own. Therefore, they have to buy a robot. In that case, it becomes almost impossible for them to determine what the robot is doing exactly and whether it will be able to keep the lofty promises made by its designer.
They have to place a lot of trust and money in the hands of something they neither understand nor control.