Paraneoptera

Legionella won't make you sick if you drink it, only if you breathe it in. It exists in clean drinkable water all over the world. It's capable of hibernation and able to survive without nutrients for long periods. So it's at least a potential risk any time you breathe aerosolized particles from water that is not hot enough to kill it. Any type of water heater that heats to 60 degrees Celsius will kill it.

This is a great idea if you are the only one using your shower. If you have 4 family members, each of whom likes a different shower temperature, it is less ideal. I think controls that allow separate on/off and hot/cold dimensions are best for most scenarios.