I find it takes a great deal of luminosity in order to be honest with someone. If I am in a bad mood, I might feel that its my honest opinion that they are annoying when in fact what is going on in my brain has nothing to do with their actions. I might have been able to like the play in other circumstances, but was having a bad day so flaws I might have been otherwise able to overlook were magnified in my mind. etc.

This is my main fear with radical honesty, since it seems to promote thinking that negative thoughts are true just because they are negative. The reasoning going 'I would not say this if I were being polite, but I am thinking it, therefore it is true' without realizing that your brain can make your thoughts be more negative from the truth just as easily as it can make them more positive than the truth.

In fact, saying you enjoyed something you didnt enjoy, and signalling enjoyment with appropriate facial muscles (smiling etc) can improve your mood by itself, especially if it makes the other person smile.

Many intelligent people get lots of practice pointing out flaws, and it is possible that this trains the brain into a mode where one's first thoughts on a topic will be critical regardless of the 'true' reaction. If your brain automatically looks for flaws in something and then a friend asks your honest opinion you would tell them the flaws; but if you look for things to compliment your 'honest' opinion might be different.

tl;dr honesty is harder than many naively think, because our brains are not perfect reporters of their state, and even if they were good luck explaining your inner feelings about something across the inferential distance. Better to just adjust all your reactions slightly in the positive direction to reap the benefits of happier interactions (but only slightly, don't say you liked activities you loathed otherwise you'll be asked back, say they were ok but not your cup of tea etc)