I suppose it depends - Azeroth has been bungled multiple times, sure; but countless other worlds have fallen with a systematic and seemingly highly victorious pattern. If the mission is destroying the current universe the Legion is winning the engagement quite handily. But given Sargeras' focus on claiming and corrupting Azeroth's world-soul, losses on Azeroth might be considered more prominent than multiple successes on other worlds. I'd imagine Mannoroth and the Legion in general have an overall great track record in the aggregate sense - Azeroth is just the foremost stumbling-block for the Legion's efforts due to its special nature.