I would argue that looking at a govt’s balance sheet is misleading. You need to judge a govt’s effectiveness by the prosperity of its citizenry. Govt doesn’t exist for its own benefit. It is not a profit generating entity. So it shouldn’t be run like one.
The ratings agencies need to be more precise in their ratings. If they are going to measure a revenue constrained country like Greece then their normal MO would apply. If they are going to rate a country like the USA, which is not revenue constrained, they should focus on the risk of hyperinflation.
http://pragcap.com/the-new-normal-era-of-debt