This strongly suggests that the best mix of monetary and fiscal policy will depend on the economic situation, and any derived optimality condition is likely to serve only as a rough guide. Rather, policy coordination around an agreed goal seems to hold out more promise than seeking some optimality condition.
The anecdotal “evidence” in each case is impressive, but it is, nevertheless, difficult to quantify these forces and therefore test their significance and measure their importance. But let’s at least make an effort to understand the stories.