I know it might be more work to burn, basically a high bark to wood ratio, and because of the smaller sizes of slabs they do not tend to hold heat as well. BUt hey FREE WOOD = free wood.
Back in the day, a local sawmill used to sell there slab and some time cut them if you were friend, family or regular who buys LOTS. There setup up was pretty nice, had a rack 4'x4'x8'; once full they wouldband it in either 1 or 2 spots depending on length, if 24" twice, if 32" once. They wouldthen bring the bundle over and break out the chainsaw with the crazy big bar (like 54" of 60") big enough to cut thru whole stack, 2 or 3 cuts and WHAM a cord of fire wood