Well Richard Koo agrees with MMTer Dan Kervick that the Fed has "shot its wad" as the President of the United States was wont to say, but Koo has some idiosyncratic views as Krugman has pointed out.

...But even at the best of circumstances, those so-called non-traditional policies achieve very little. I mean, probably better than nothing, but just a little of positive, whereas having the correct fiscal policy can have a huge impact on economy in the current circumstances.  Because so many people are pinning their hope on the federal reserve, there's little discussion on what is the right fiscal policy and that I think is very unfortunate.

And the second part of the problem is that if central banks are viewed in such a way that these guys are going to pump money into the system so that something happens to the nominal GDP, that can cause people to worry that if they're gonna pump that much money into the system, the dollar may collapse. And that can cause another set of problems like foreign investors dumping U.S. treasuries or something because if they know that central bank has limited capacity but are forced to do something to get the nominal GDP going, people might assume that "Well then, the central banks might really do something crazy," and thats not good for the credibility of the dollar or the central bank.