It was the problem, because doing REYES hiding you suffer the huge "overdrawing" issue, where you need to set the shading rate per object in the scene based on its size on screen such that it doesn't get too much shading. If it had too much shading (the shading rate was too low), many more rays will be sent. And this is because REYES shades stuff before it sees if it's visible, whereas raytracing from the camera (with the raytrace hider in PRMan) only shades the points which are visible, so it scales much more on complex scenes (when GI comes into the picture - without GI REYES can cope easily as it pages everything).
The main problem with RenderMan before version 19 was that it didn't have a good acceleration structure and that you basically had to implement a path tracer in RSL on your own. Pixar wrote a path tracer in RSL for Monster's University, so Disney could have just used that. I doubt that they would write their own renderer if they could have just asked Pixar to implement a better acceleration structure in PRMan, so there were obviously different reasons too.
I think Ptex was an important reason for them to write their own renderer, as they seem to put a lot of focus on textures in their paper on the architecture of Hyperion [2]. Hyperion's architecture also allows them to use packet tracing and to load and subdivide geometry on demand, which is also really great.
[1] http://www.realtimerendering.com/resources/RTNews/html/rtnv1...
[2] https://disney-animation.s3.amazonaws.com/uploads/production...