Well, this may be a subcultural distinction. In my circles, saying "Jeez oh man are you wrong, let's get coffee to discuss" is affectionate, but it doesn't mean people pull their punches over coffee. So fondness doesn't require soft-pedaling.

I find it easier to look for weak points when both people are enthused to take each other on a tour through their beliefs (ideological show and tell, basically). If the discussion feels really framed around "discover weak points" it's often harder for people to reveal them, because it feels like once they're spotted, you'll be laughed out of the argument. Alternatively, you can create a context where it's easier to say, "This is the part that I find a bit confusing myself, to be honest, though I still am relatively confident in my overall model. What would your model say about this sticky widget?"