Why is the US complaining about the disputes resolution system? They have won cases while Canada and Mexico have won none.

I doubt the settlement track record will be any better for US businesses that have to go through the courts to get satisfaction. The courts will take far longer, in some cases decades to settle. Resolution is less certain than with Chapter 11. 

With respect to the Nova Scotia case, the US company may have a case that it was mislead by municipal officials but I think it is asking for too much. A rock quarry is not that big cash generating business.