They put your driveline at a horrible angle with anything over a 3inch block. They cause major axle wrap. They have a tendency to shift if u-bolts are not tightend down regulary. I've seen people wheeling with lift blocks that the block slipped out from the axle and leaf spring.
You will have not flex with a block between your leaf spring and axle. If your building up a truck, why half ass it? It's one thing to use a 1inch or a 2inch block for a little extra lift if needed, but I don't see why you would. Leaf springs or a shackle flip are a much smarter way of lifting a rig.
Huge redneck blocks like that are not safe. Even if you weld it to the axle, why bother? Run lift springs or a shackle flip. Lift blocks up front are not safe on a solid axle truck. Double stacked blocks are not safe. Lift blocks are not safe for the most part. And with a short wheel base like 2 doors and 4 doors, the axle wrap will get worse with blocks then on like a ECSB or so. Jeep guys don't run blocks for a reason...
I know people here like body lifts too. I personally hate body lifts, but I'd run a body lift before huge redneck blocks
Again, this is for huge lift blocks (3inch or up). Biggest I'd ever run would be a 2inch.