A couple updates ago Bosses were given immunity to stuns and knockdowns, and the reason given for that was in the old dbo that's how it was. Here is proof it wasn't like that.
@2:25 - 2:38 you see regular stun and paralyze land on a Boss.
Here you can see SM spamming kidney shot on a Boss. (Credit to adrian on fire for finding the videos)
You just needed very good Focus, hence why the SM was doing it. But this did not mean they had high resistance because debuffs landed on them well, they were just more resistant to KDs and stuns. I can't find proof on KDs working on bosses but they did, maybe if someone could help find a video with proof. Speaking of KDs, I think i'm the first to mention this but KDs are too short on mobs. They last for about a second but in TW it felt more like 2.5 seconds.