Is training to failure a safe and effective method for improving athletic performance?

I don’t remember reading in any text book that you should train your client until they can’t sleep…? While this may get results in a few situations I find it hard to believe that training to failure is something many clients will be able to do without psychological and physical strain or injury. Train don’t strain! While I can’t say it’s impossible it doesn’t seem like the reward outweighs the risk.