For any aspiring interrobangers, one thing that helps make them easier to use is to set up text replacements. I have replacements setup on my phone and computers to replace “?!” and “!?” with “‽” automatically.
?! is about first questioning and then usually being angry.
!? is about first having strong feelings and then questioning their origins.
So for me WTF would usually be followed by ?! but maybe it's just me.
WTF?! with an emphasis on the question means you lack information about the subject, WTF!? with an emphasis on the exclamation means you know what occurred but you're expressing shock/outrage.
E.g. the first version is "I don't know what just happened" and the second is "I can't believe what just happened".
With an interrobang there's no way of expressing both versions.