Are frost fans bad?

Definitely not.

They are (in my humble opinion) an effective form of frost control.
However, they are REALLY NOISY!

So noisy that when placed alongside people they are more than a simple nuisance, they can and do prevent sleep and adversely affect health.
The legal beagles can argue as much as they want over what constitutes a 'reasonable noise'. The fact of the matter is that some people will not be able to sleep with a wind machine rotor turning through the night outside their house (seems simple to me, but I'm a simple person).
When that person tries to function after a night of poor sleep, they will not be capable of performing to the same level.
Now that might not sound like such a big deal, but what if that person drives to work?
What if they drive a taxi or a bus?
What if they operate heavy machinery?
What if they are a surgeon operating on you?

Frost fans are not bad, but their effects can be if they are not managed properly. It's the irresponsible use management and monitoring of them that has given them a bad reputation.