It's the debt they saddled the club with, they couldn't afford to buy united so borrowed and then transfered the repayments to united, the rags arguement (well the ones who have been doing this for a decade now) is that no loan repayments and debt well that money would be reinvested into the club.
If the club suddenly lost revenue and stopped turning a profit then they would go under as financially they are fucked if unsucessful.
We would hate it if own owner had done the same tbf.
However they still compete and the debt is managable and because of their global brand the likelyhood of revenue dipping to a point where it is an issue is slim so they have fuck all to moan about, and insult fans of bury, pompey, chester, macc etc when they compare their situation with them.