In the period of "low for long," many central banks faced the effective lower bound of policy rates, which is the traditional policy instrument, and have created various unconventional policy toolkits, including large-scale asset purchases and forward guidance.
While a number of empirical studies support some effectiveness of these instruments in stimulating demand, it seems that the mechanisms at play are not fully understood even today. Compared to conventional instruments, which have a long history, unconventional instruments have issues such as limited experience and data constraints in measuring their effectiveness, and further theory building and testing will be required.