Duck can taste ok cooked thoroughly, but it should always be cooked low and slow. I think you missed the mark by cooking it on high for so long. Keep in mind, they are small pieces of meat.
In my opinion, it's tough to do traditional roast with duck. When I use a slow cooker for duck, I've had luck shredding the meat when able, adding more moisture (bbq sauce) and letting it soak it up for a little longer.
If you like using it, try a "pulled pork" approach with the crockpot, substitute any game for the pork. McCormick makes a decent seasoning packet that makes the approach simple and tasty.