I had IVRO, and I'm a class II (wish I could say "was," but I still am, unfortunately). The reason my surgeon chose IVRO was that it is less traumatic to to the joints (he explained it, but I can't remember exactly why now, but I think you could google it). The other main reason was for asymmetry. He said IVRO is better for severe asymmetry compared to BSSO if the mandible does not require forward movement. I believe when he did the posterior impaction of my maxilla, I had enough auto-rotation to correct the class II. He knew that IVRO was a possibility, but even after the model surgery, he was not entirely sure I'd have enough auto-rotation. He did the maxilla first, and then made the decision for the IVRO.
I think the two big drawbacks to IVRO are patient discomfort (it is not fun to be wired for six weeks) and stability. Muscles are strong, and there can be bony shifts when wired. This is less so the case with rigid fixation.
Yes, IVRO is primarily for underbites, because it is not possible to bring the mandible forward with IVRO, but it is possible to set it back to correct an underbite.
I have numbness, but I think it's from the genio.