Just adding people to a project usually leads to delays and code that has to be refactored later.
However, it is almost always a management issue. It is management that brings a finalized product to market and approves everything that goes into it. Since Blizzard follows a principle of just get it out and polish it later, there will be problems. They haven’t shipped a product “when its polished and ready” since Wrath of the Lich King was originally released.