gear drive

I was looking at a JEGS magazine and was looking at a gear drive, below the advertisement of the gear drive it said that it is not for use with vehicles that have a hydrolic cam, several other gear drives had the same disclamer, why cant you use a hydrolic cam in combination with a gear drive?

Not sure. I've heard that some shouldn't be used with rollers since the gear isn't machined for a thrust button, but never heard about hydraulic problems.