uh
you're right, magnetic fields cannot affect them, but I read this big write-up on how CD-RW's are way less stable than CD-R's. Apparently they last far less amount of time, due to the fact that the reflective surface is very fragile and starts to break down very quickly. They don't use organic dye, like CD-R's do.
Apparently there's a new type of high-speed CD-RW disks about to come on the market that do not suffer from this limitation, and will also allow higher-speed CD-RW burns to take place. Right now, the highest speed you can safely burn at with a CD-RW is 4x, regardless of your drive.