The main problem with using sawdust around plants is that it will pull the nitrogen out of the soil, which is bad for the plants. The critters that break down the sawdust take the nitrogen for themselves to do it, reducing what is available to the plants.
If you dump a 55 gallon drum of sawdust in the woods, you'll be surprised how long it takes for it to compost. I've got some that have been there for years. I've heard stories of sawdust piles in northern MN left over from logging being around for decades. Granted that was white and red pine, which are relatively rot resistant, but you get the general idea.