Just reading this thread over lunch before getting back to some leather work so I threw my piercing tools out for a quick photo.
The little skinny awl with the knob handle in the upper right does the great majority of sewing for me. This awl is some sort of industrial needle set into a handle, made by some friends in Oregon. The fat handled one in the lower right is a book binder's awl from Turkey. They repurpose old dental tools for these. The other two on the right are a little fatter taper for thick stuff. The upper right I made myself from a dental tool and the lower was inherited from great grandpa. The big woodworking awl is good for hole-stretching and the curved cordwainer's awls are vital for lining up holes through many thick layers. The skinny punch at the top is great for clean, round corners and of coarse, the rotary punch does everything else for lacing and riveting.
Like the man said, keep them sharp. There is no limit to thickness but better and straighter runs are generally had if you can punch the layers individually.
Almost forgot: the lumps are beeswax. Vital for vegetable tanned leather. The awls actually get quite hot when you are sewing fast.