There is a marked difference between being
a) polite and compassionate; and
b) cowardly.

I too used to hide behind the concept of "niceness" to justify conflict-avoidance. Not that I don't still hide from conflict, but I don't dress it up as a virtue anymore.

If I call them on it but am nice to them later? Well, I may still be being complicit (I'm still thinking about that one)

I think you can disagree with someone and still be friends, or at least friendly, as long you don't excuse them simply because you're friends.