The first is the implicit assumption that the lags are much the same length from episode to episode (occasionally there are tests for changing lag length, as in Gruen et al. (1997)), whereas it may be that the lag depends very much on the particular episode.
Where actors in the economy quickly come to believe that the authorities are determined to slow a speeding economy, it may well be that the lags are quite short. The contrast here is between 1988 (long lags) and 1994 (short). The other problem is the classic one of separately identifying the policy reaction function and the effect of policy on the economy.