I've been looking at this reaction for cleavage of benzyl esters. I've also looked at the original paper and some others, but haven't been able to find any mechanism on how this reagent does hydrogenation.
It also works with Cbz-protected amines/amides/alcohols. With palladium/platinum, the transition metal does oxidative insertions into the X-Bn bond, but I'm not sure nickel could do this insertion. If it could, I would expect this reagent to debenzylate amines, amides and ethers in addition to esters, carbamates and carbonates.
I was wondering if it is known how this reagent debenzylates esters or if anyone has any insight into the mechanism.