The early evidence suggests that it does work to some degree to stimulate the economy, although the precise mechanisms involved are still a matter of some debate.
The large expansion of the central bank’s balance sheet under QE markedly increases the central bank’s financial risk, and its dominance in the targeted markets distorts market pricing (indeed, the distortions are one means by which QE is believed to work). These factors place limits on how much QE can be relied upon as an additional tool.