In spigot, when a black wither skull hits a block with blast resistance below 20, the block gets destroyed but does not always drop as an item. In vanilla however, the chance that the block drops as an item is 100%. This is making black skull based automatic tree farms extremely inefficient.

Attached is the comparison of item-dropping chances in vanilla and spigot. A 3*3*4 cubic of dirt is bombed with black wither skulls until all dirt blocks are cleared, and items are collected after each explosion.