BVB can beat anyone in Signal Iduna Park. They outclassed Bayern in 2010-11 and 2011-12 Bundesliga.
But given Bayern's current form and the final being at Wembley, a lot of supporters need to travel to give them a chance.
Also, both of them played a preview match yesterday which turned out to be 1-1. But imo, Bayern played a weaker side as they have already claimed Bundesliga.