I never understood why people disregard her voice in the studio. If it's not her voice who's voice is it? It's not Miley's voice i don't think. I don't think it's Leona's voice. It's not Celine's voice.
Because a person's voice can be manipulated in the studio in such a way that it sounds more impressive than what the person is actually capable of.
Case in point. She's no way, no how capable of doing that live.
In truth, most any singer's studio performance is manipulated like this at least insofar as adding extra resonance. You never hear just a straight mic feed when you listen to a pop performance, anyway. Even something like that Whitney performance of How Will I Know isn't just the mic feed; it's the post-production vocal track.
This is what Mariah sounded like in 1997, for instance, on a straight unenhanced mic feed. Compare it to the final studio performance, and you should hear how her voice seems bigger and, well, enhanced. You can hear the damage in her voice much more apparently than you can hear it in the final studio version. You can even hear the part, at the climax, where two different runs are edited together which is sort of disguised in the final version!
And her voice is in better shape than it is has been in awhile, but you need to be drinking a special fanboy Kool-Aid to believe that the higher belted notes aren't the result of multiple takes and vocal layering. When she sings notes above C#5 live these days they tend to be very rough and that performance of The Art of Letting Go is a great example of that issue. Granted, there are some exceptions to the trend; for whatever reason she sings impromptu notes (e.g. Love Takes Time) or I'll Be There (the C#5s/E5s) much better than she sings them in other songs.