If you are using a sling to attach it to the grapple truck where do you attach the sling? there isn't an attachment point for it. Is the sling rubbing the metal somewhere that could cause it to be cut. with the weight of the log metal can easily cut through a sling. The grapple is what should be used. Slings can also slip off with loose or springtime bark. I have seen it with a crane lift before also.
This was a long debate on the CTSP Facebook group. Modern log trucks have check valves. The final word from the manufacturer was the design of the stabilizers. The aren’t engineered to reach out high and pick stuff up I forget what manufacturer that came from. Major one though. That was the only reason the didn’t endorse that use. Be smart about it and you won’t get dead.
I wouldn't do it for a few different reasons. I keep 2 large shackles and a couple slings on the truck if we have to pick something vertical. But can't remember the last time we had to use them. Flop it and pick it up.