I've given some thought to AWD bikes and have come to the conclusion that they would only be useful in winter, on snow-covered roads. The best way, IMHO, to implement AWD is to replace the front wheel with one of those electric hubs so that it's basically a front-wheel-drive e-bike but with rear wheel driven by the cranks. I have never tried this, but I think it might provide some advantage in slippery conditions.
Luis