Hi Tristan, you are right, the haas effect is known to cause phase cancellation issues. That's why I wouldn't recommend using it on critical elements of your mix if you are concerned with the listening experience of people who might listening to it in mono (this might also depend on who your target audience is. Due to phase cancellation, certain leemnts might change in volume, tone or disappear in extreme cases. However, I think the effect is still useful and it is frequently used by a lot of great sound designers and producers; you just have to choose the right elements to use it on and frequently check your mix in mono. :)