Since the method uses actual real rates it already inherits the arbitrariness of measuring them: one has to decide between ex ante and ex post measures of inflation and on the type of price index used. Additionally, one has to decide the window width over which the sequence of means is computed.
Since all these choices will have an influence on the level of the estimated natural real rate, different variants of this approach can result in quite different point estimates of the NRI. The NRIs resulting from such moving average estimations typically constitute a fairly smooth time series.