My problem with the interrobang, besides the fact it's unnecessary ("?!" does just fine, the same way we don't need to replace "th" with "θ"), is that it's so damned ugly and hard to distinguish.
The counter [1] -- the open space between the vertical bar and the question mark curve -- is just too cramped ("‽"), so it's aesthetically unbalanced at large sizes, and just looks like a question mark with an accidental smudge at small sizes. It just doesn't "fit in" with any of the other glyphs of the alphabet, it's badly designed.