I have never smoked with roots, but I often run a stump grinder as part of my business. While grinding, some roots have a significantly earthier odor than the same tree above grade. I would do a test burn in my fire-pit before committing meat to the mix. Sometimes, earthy is a useful flavor component.
Even after pressure washing, the roots likely hold some sand in the bark and sometimes even in the wood itself. Be prepared to sharpen your saw or axe far more frequently than above ground.
18.5 WSM, Green Weber Genesis320, Cast iron firepit(w/grill), and a Copper Weber 22" OTS