As a former employee of McDonald's, imma let y'all in on a secret. 95% of the time, they work perfectly.
People just happen to want ice cream during the other 5%, which is: the machine is overheated(ironic, isn't it), it's being cleaned(can take up to around 4 hours), or it didn't get filled properly overnight, and it essentially resets itself.
Also, most machines have 2 sides. One for ice cream, one for shakes. Usually at least one of the sides is working. Just ask.
I only remember our machine being actually broken once, because it needed a new part.