The brakes can handle rain, no problem at all.
There are two problems, though, and both have to do with rubber tires. One problem is that the anti-rollback release generator will slip on the lift hill and not properly disengage the anti-rollbacks. This will cause wear on the anti-rollbacks, but it is not a safety problem. It just makes the Millennium Force lift almost as loud as Magnum's.
The other problem is a bit more serious. A difficulty with magnetic brakes is that the braking force is directly proportional to the speed of the conductor (brake fin). So when the speed drops to zero, there is no braking force. This means that the train is not stopped by the magnetic brakes, but rather by the four kicker wheels on the brake run. Yup, we're back to damn kicker wheels again. Why don't European rides use holding brakes and gravity like most American rides do? If a kicker wheel is wet, it won't stop the train, at least not without slipping, and that's a huge problem.
--Dave Althoff, Jr.