I can understand the desire to use one for load development when shooting a really heavy recoiling gun. If I had access to one I'd use it for this. I wouldn't use one for zeroing any gun because the gun may well shoot to a different point of impact when using a different type of rest.
I believe it is best to do all zeroing and practice off something as close as what you may use in the field as possible. Shooting off an improvised rest, or even sandbags develops skills not learned when using one of the sleds.
Even with sandbags as a rest there is movement of the sights on the target just as in the field. Becoming a marksman is knowing exactly how much pressure to apply to the trigger as you anticipate when the crosshairs will be exacty where you want them. You never develop those skills when shooting off a lead sled. In the long run you will become a better shot without one.
They also put a lot of stress on rifle stocks. Since there is no "give" when fired, the stock takes all the stress. It is not unheard of for stocks to break when fired in lead sleds.