Having seen the Suramar story in its fullest, I can completely understand the Nightborne's decision to join the Horde.
Tyrande's lines towards them, and about them is pure spite. I can understand her distrust in them to begin with, but over the course of the insurrection story line, there were many points where the Nightborne proved which side they were on, and were always very forward about their overall end goal after the fall of Elisande. This was the place of her birth for crying out loud! I know over time the Night Elves have shifted away and become their own entity entirely, but to forget ones heritage, is to forget ones sense of self.
And the stuff she says during the Post Notice WQ is just despicable IMO.