The real risk here is if inflation makes a reappearance, say as a result of people starting to spend all that money floating around. As I understand it the way

The real risk here is if inflation makes a reappearance, say as a result of people starting to spend all that money floating around. As I understand it the way to deal with that is to raise interest rates, which means that the cost of servicing all that debt suddenly increases for everyone that holds it (both the US government and private individuals). This could be really painful. In this scenario, the problem is the combination of both the large amounts of debt and inflation rearing its head; either one alone would be manageable.