The article mentions how undercover detectives can burn out. I have heard that this happens quite a bit. I saw a movie that was based on a true story where the undercover cop got to the point that he felt more at home with the mobsters he was investigating than he did with his own family and the cops he worked with.
This type of job has to be mentally exhausting. I would be constantly concerned that I was going to get confused and say the wrong thing.