Rails in general have lower magnetic pull due to the construction vs. a single coil with magnetic pole pieces.
It's probably usable in the middle with a full-size humbucker, but I'd only do that with a very hot bridge humbucker, like a D-Activator X or X2N. It's got similar output to something like a Super Distortion, so it's already hot even by humbucker standards, and I would be hesitant to pair it up with anything that isn't even hotter than that.
