Yeah, but most people cannot take constructive criticism, whether they think of themselves as being professional or not. They might put on a good face but are internally seething that someone dared critique them. It's a difficult line and if you need to give that sort of feedback to someone because you are actively working together, that's one thing. People can be professional, as you say, and can work through it.

But if you are leaving a company, that's even less of a reason for the person you're critiquing to listen to what you are saying (b/c they can just write it off as someone who has sour grapes or was leaving anyway), and even more of an opportunity to just create unnecessary animosity. And that professionalism that becomes a necessity when the person is still a coworker can disappear completely.