I used to fill out the comment cards on board to highlight good crew on a regular basis. However, on a JNB-DXB flight a few years ago, I was confronted by a Purser who basically accused the crewmember and myself of some sort of connivance to garner a favourable review. I was "given the opportunity to withdraw" my comment card, which I declined. I was subsequently contacted by someone from Service Audit who asked similar leading questions in what appeared to be an attempt to pin some sort of wrongdoing on the crewmember with respect to soliciting compliments or similar.
Since then, I no longer fill out comment cards on board. My various friends at Emirates also agree that a handful of Pursers take it very badly when individuals are singled out for praise and that can often backfire on the crewmember you are trying to praise. It is far better to email directly AFTER the flight than to fill up the cards on board. Just something to keep in mind.