What he said...^^^
Of course, if you use a DAW, it's just as simple to edit out the vocal "esses". Yes, it take more time, but you can be very surgical, and make each edit specific to a particular "ess" problem, since not every "ess" sound will be the same or always hit the same frequency with the same force.
An actual de-esser will just act on what it's set at...while you editing, can tailor it to individual problems.
I usually have a 2-3 parametric EQ presets with narrow bandwidth cut, set to my most common "esse" problem areas...then when I need to de-ess, I select the range/length of track I need to correct, apply the preset, slide it to the most effective frequency point, and adjust it's amount of EQ cut.
It sounds like a lot of work...but it's really not...and the de-ess control is much more precise.
Sometimes, all you need to do is select the range/length of audio in the track...and simple lower the level of that sections...no EQ needed.