I think the answer is, it depends. There have been complaints of stocks cracking in heavy recoiling rifles because the lead sled does a little too good a job of providing resistance. If you calculate the amount of distance a free recoiling rifle moves back while the bullet is still in the barrel, it's on the order of a sixteenth of an inch for common hunting rifles, though an elephant gun that has a long barrel time would go further. My approach has been to pad the butt sling on the lead sled with a Sorbothane pad I picked up at one of the NRA annual meetings. But a Neoprene foam pad or even a couple of layers of medium pile carpet scrap should accomplish about the same thing: preventing a truly hard stop.
The other time the lead sled becomes questionable is when you have a rifle whose barrel doesn't float in the stock and which doesn't have pre-loading of what contact points there may be. It is not infrequently the case that you will disturb POI in such rifles less by resting the magazine well or, in the case of a lever gun, the receiver just under the carrier on a bag instead of the front end of the stock. The lead sled designs I've seen thus far don't allow for that variation on sandbag technique to be applied.
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