Why are straight legged deadlifts not a good idea to do?

The key to any exercise is to do it with correct form, and a proper amount of resistance. Like Karin said, the exercise is only wrong if it is contraindicated for the client. If you want to get technical about it, regular exercise is always associated with a higher risk of injury. Every trainer has an opinion of what is appropriate for his/her client. I would argue that it’s often the case that most exercises can be safely modified according to the needs and abilities of the client.