Ultimate abilities are called ultimate for a reason
So it makes sense that the cases of Malphite's, Vi's, and Warwick's ultimate abilities should be immune to Poppy's W. However, Warwick's Q should not be granted that same immunity. It damages, heals, and applies on hit effects while also being able to latch on to every champion in the game through flash and all movement. Her W should stop it as a special case against its displacement immunity, not the other way around.
Don't know why people would hate on one interaction for Poppy when she is neither overpowered nor overpicked right now.