It definitely goes both ways: the Kaldorei would never believe Illidan because of their own biases, but of course he never bothered to tell them…because he knew they wouldn’t believe him.
The important difference here, though, is the fact that of the two, only Illidan actually bothers to engage in introspection. Not one do you ever see Malfurion or Tyrande thinking, “Huh, maybe we were wrong about Illidan. Maybe we should have been more tolerant.”
Meanwhile:
What if he was wrong? What if he had miscalculated? His visions were not infallible. Perhaps there was another way and he had not seen it. Perhaps he was blind to a possibility that might win this war without all the sacrifices. He had searched and searched for one and had not found it, but that did not mean it was not there.
World of Warcraft: Illidan, p. 253
If you really think about it, if you put Illidan up alongside pretty much every other major character in the series, this level of self-questioning is kind of unprecedented. Everyone, from Jaina to Anduin to Thrall to Sylvanas, we always see acting in complete confidence of themselves–even when it comes to things like plaguing Gilneas or flooding Orgrimmar/purging Dalaran, rarely if ever do we see any of these characters actually taking the time to ask, “Am I the doing the right thing here?”
That’s ultimately what, in many ways, makes Illidan Stormrage a far better-developed character than anyone on Team Red or Blue. Illidan is quite probably the only major lore character, at least that I can think of, who actively engages in self-doubt and introspection, but still chooses to forge ahead.
Not because he’s ignoring the possibility that he’s wrong, but because he genuinely believes that he’s doing the right thing, in order to save as many lives as possible (remember, we’re talking on a universal scale) in the long run.