Not telling the entire truth during the very last phase of employment, the exit interview, is but a drip in a bucket of lies that is called employment.
You lied to get in. By emphasizing your strengths and hiding your weaknesses. The employer did the same thing.
You lie constantly about the work or its conditions being fine, as complaining about it is kind of a taboo. You lie to your peers and superiors all the time when they do or say things you disagree with, especially when the cost of conflict is too high.
You lie about the company drink being so much fun, whilst you really don't want to go. You lie about how much effort that task really costs, as you want to build in some security. You lie about how much time you're not really working.
You lie about being a well behaved human being whilst the minute you arrive home, you burp, fart and swear all over the place.
...yet you won't at your mother-in-law.
Conclusion: any human relation that is involuntary (you do not directly pick your manager, colleagues or mother-in-law) is diplomatic, which is a fancy word for not genuine. You keep up appearances and tell people what they want to hear, because the downside of the truth is too large.
So silence or lying is fine. Unless some massive humanitarian thing is at stake based on your input, which I figure to be rare. Alternatively, one can learn how to criticize in a non-damaging way.