Last fall, my small group did the hills on pavement. I don’t think it created any problems since the landing surface being inclined limits the stress on the leg since there is such a small distance for the foot to fall and come in contact with the surface. We simply did not have a grass hill that was shallow enough to run on. If we had we would have done the work on grass.
Moving from hill work onto the flat surfaces my guys did the vast majority of this work on the grass and I’m definitely going to do it again this year. I feel like they certainly developed speed and they stayed very healthy. We experienced very few injuries/irritations.