A lead sled is a perfect recipe to destroy a dangerous game rifle. Sure, to soften a 30-06 its not a big deal, but the bigger you go, the heavier the weight added, and the stiffer the recoil of the load, the more likely to do tremendous damage. Breaking a $4000 scope is actually not the worst thing that can happen, breaking the wrist on the rifle could be a $15,000 mistake that takes years to resolve.
If anyone thinks this is nonsense, try this: take your largebore rifle out and stand it with the butt against a brick wall. Fire the rifle three times. Then cry. Recoil has to go somewhere, and a brick wall is not going to yield. Your stock, scope, and mounts will. An overweight lead sled is just a brick wall of another sort.
Adding to all of this, for largebores specifically, the recoil impulse out of a lead sled is different enough than shooting freehand off sticks that your point of impact is significantly altered.
So in conclusion, for big guns, a lead sled does not reproduce real-world shooting conditions, is likely to break your rifle, and will not be field-repeatable point of impact.
I hate lead sleds.