Thank you in return for thinking through my suggestion :-)
Everything you said sounds fine, with the exception of what makes front-stabbing, front-stabbing.
I agree that maturity is important for giving constructive feedback; just as important, in my opinion, is closeness. That closeness is certainly achievable in a small startup, but the company scales, I have strong doubts that it will match the level of honesty Empathy, the company value, demands.
More to the point: when unleashed in a social context where all parties aren't friends, I believe that "true" honesty (concerning a person's work) will inevitably transform into front-stabbing (attacking a person via their work) .
If someone who I was not close with was uncomfortably honest to me in the middle of a meeting, in front of many other people, I would probably ask myself "Is this person saying this because I'm actually doing something wrong? Or is he just trying to make me look bad?"[1]
When many ears are listening and many eyes are watching, I know I would feel less inclined to reflect on her or his words, and more inclined to worry about my loss of face. Especially if this person has made a habit out of being vocally honest with others. In my opinion, maturity isn't 1:1 with morality; a person can act like an adult and also be ruthlessly ambitious enough to take advantage of and hurt others.
The deep, deep end of being uncomfortably honest among others in a workplace is a struggle session, where one person is in the middle of the room being berated by everyone else. No one wants to let it go that far, at least not initially, but I certainly believe the basic social lubricant of politeness must be valued, and applied in most contexts[2]
Privacy, maturity and closeness are the makings of a real heart to heart, where both sides can learn from each other. Missing one, or having any in deficiency and, as the company scales, the risk of front-stabbing grows exponential[3]. Said another way by a Wall Street Journal commenter: "You better not front-stab your boss."
[1] There was an article recently that surmised that all public apologies are purely P.R; any demonstration of heartfelt guilt is quickly exploited as an act of weakness, so the apologizer is as tightlipped as she or he can be. Can't seem to find it...
[2] Of course, the other deep, deep end is a Japanese firm where the knives are under the table instead of on the plates!
[3] It's 1:40a.m. in NYC so I hope I'm coming across as explaining my perspective instead of as some kind of internet troll.