From a bad past experience of telling the wrong personality my honest but tactful opinion, I can say you should only harp on the positive, even if you have to compliment how well the crew is sweeping the floor. Especially when the director is playing more than one role (actor, producer, playwright, etc.) as feelings are bound to get bruised no matter how nicely you state your opinion. You can't take a chance, no matter how good of a job you do, that the only thing you'll be remembered for is negativity come time for rehiring.
That being said, I have had several directors ask my honest opinion on specific aspects of the show, usually because they need another perspective. In that case I give my honest opinion, but try to keep it simple and don't elaborate, and they have all appreciated it. If they ask me the general question "What do you think about the show?" - I'm always on the positive.