The more restricted travel could induce much more extreme vibrations both to the rifle's internals and the scope's. That recoil energy has to go somewhere and stopping/reducing it at one point will cause it to be distributed elsewhere. I think the damage both to stock and scope internals is a valid concern. And I too have read quite a number of reports of scope failures while shooting from a lead sled.

Disclaimer: if the above post makes it seem that I have the slightest clue as to what I'm talking about, I apologize in advance.