Agree. Producing continuous power is a lot different than intermittent short term power. It will certainly depend on the characteristics of where you ride, but in general in my experience, the road bike demands continuous power levels as it's the power that limits how fast you go whereas on the mtb the power required fluctuates much more because of the technical and varying aspects of the terrain. Sure you may have a long relatively smooth uphill that's all just power on an MTB, but that's relatively unusual in my MTB riding.