In any situation, you don't want to be the lone wolf of honesty. Your company or team needs a culture of honesty. It's simpler than you realize. Have a conversation, make an event out of it. Say, "Look, for this to work, we have to be outspoken and honest. Our success depends on it. It will be uncomfortable at times. It will be frustrating at others. But there's no reason to invest this much, to work this hard, just to stop short of honesty. If you can take a startup, you can take honest feedback. And we should all expect a ton of it."
The TechStars program does this really well. They promise you brutal honesty. They tell you to brace for it. You are guaranteed to be told when your idea sucks. You can see this in practice each week as the teams present their pitches. Because honesty is the expectation, the quietest guy in the room will raise his hand and say, "For the first ten minutes, I had no freaking clue what you were talking about." Ouch! But the permission was there. The guy wasn't a jerk, he was embracing the culture. And you know what? It needed to be said.
In the early days, Rackspace had a saying: "Bad news first, full disclosure, no surprises." More than a slogan, it was permission to be honest. People would say, "It sucks to bring this up, but as we say, bad news first and full disclosure..." Again, amazingly effective.
People want to be honest. And in the end, they want you to be honest, too. There just needs to be permission.