Now, it is generally taken  by rational economists without an agenda (and even most of the honest ones with an agenda) that unlike any expenditure being good and a possible source of "aggregate demand" that rather than as Lord Keynes promoted in saying any expense is good (in effect even war) that:
"Government spending should be set at the amounts of government goods and services that the public would demand if the economy were at full employment; no make-work projects should be permitted."
This is the flaw. The assumption is that government can choose better than the market. This is what this entire assumption comes to. It states that a small group of bureaucratic public servants with little access to a real business or market knowledge -limited knowledge - can out decide a market. Any market this is.
This is the flaw. If we are looking at Krugman or any other neo-Keynesian, what they do not want you to know is that they are basing the policy on an idea that central planning works better than a market. That a small group of government employees can at all times beat the combined knowledge of all businesses in the market and that the consumer is wrong when they choose differently.
When you see so call support for aggregate deficit spending by government that is designed to "stimulate the economy" remember that all they have done is spend the future to make things politically better now.
Well, at least we have moved on from thinking that building pyramids and digging and filling holes is productive, but not a long way.
Lord Keynes made a system that was designed to keep Keynes a Lord. Little more.
 Wray, Government as Employer of Last Resort: Full Employment Without Inflation:
Mosler, Full Employment and Price Stability
See also Neo-Keynsianism and even Monetarism