smart:deess was not designed to reduce the harshness of instruments. Unlike conventional de-essers, smart:deess does not simply look at the signal energy in specific frequency regions - but the plug-in actually identifies the exact position of individual sibilants (e.g. S, Z, SH, CH, ...) and plosives (e.g. P, T, K, ...). Obviously, this detection of phonemes only works for voice and smart:deess will not be able to detect and process any (meaningful) phonemes in instruments.