As far as I know, there is nothing wrong with slab wood, per se. However, two issues do come to mind: 1.) it's probably pretty green, as most of it will be from freshly harvested timber; 2.) most of it will contain more bark, etc., than heartwood, so it will potentially have less heating value than traditional cord wood.
Understand that creosote is far, far more a function of how well or poorly a wood supply is seasoned - not a function of the species. Creosote is a problem caused by excess moisture and/or poor burning practices (e.g., cooler, smoldering fires).
So, if you want to use slab wood, get it now and get it stacked in the dry and you should be fine.