I think the issue is just that they kept rates too low for too long and failed to rein them in a bit when things were recovered from 2008. A little extra boom

I think the issue is just that they kept rates too low for too long and failed to rein them in a bit when things were recovered from 2008. A little extra boom probably is a net positive long-term since it facilitates innovation and growth for a time. But if you selfishly leave the spigot of free money on for too long you get complacency and bubbles. The whole concept is intimately related to the keynesian idea of stimulus spending. But even keynes thought that the government needed to walk things back when things were booming.