I really do like my Lead Sled when shooting my .470. I've had no problems with it at all and find it helps me fine- tune a load with much more precision than without it.
In my case, I only use one 25 lb. bag of shot, while it is designed to accommodate more. I've never had anything happen that resembles your situation.
As for comparing the Lead Sled to shooting a gun against a big oak tree, I see no relationship in the two. I fully understand the need for recoil when shooting a gun, not to mention protecting the stock. That energy has to go somewhere, and if against a tree, the gun is absorbing it all.
In the case of a Lead Sled, particularly when it is not severely anchored, the transfer of energy continues to take place just as if the gun was shot by someone bulkier than me. I see no reason to think that will materially impact the gun's accuracy, and based on my personal experience, see no downside to using this setup.
I like really do like a Lead Sled. To me it's a great tool for big bores that need precision, such as when experimenting with small changes in a gun’s load.