IR, old fashioned chaff may be filtered out from a modern radar processor but any chaff still blocks radar pulses, blocks them outbound and again blocks skin paints returning through the cloud.
If chaff is sown as a curtain and kept sown then it will work as an effective block at a given frequency radar. What then becomes important is that the radar system is aware of the chaff degradation.
In the self-defence role then static chaff may be processed out effectively but you should never ignore curtain or confusion chaff.