I have done this twice........and had no guilt in doing so, in these two instances. The crew works hard, 99.99% of the time.
The first was: the waiter and his helper, argued constantly, so our service was terrible, and this was HAL too! Quite a surprise, to say the least
The second was a room steward, who cleaned the room, like a teenager would, sorry, not good enough.....
I think when this does happen, I think the purser's office, should ask why,and notate the complaint, so it won't continue.