For me, something that's kind of bothered me is the way she's been singing "ahh (あ)" and "ehh (え)" since POM. To my knowledge it's because she went through some sort of vocal training before POM right? It makes her sound... almost duck-like dare I say.
I sometimes feel like her voice wasn't the actual issue (though it's arguably gone pretty worn out over the last 2-3 years), but rather the change in her singing style. If you listen to her Who performance from CDL 12-13, even though she sang it the post-POM way for the first part, she started sounding like her old self as soon as she began crying and choking up - and I think it's because her choking up made it harder to do those exaggerated Ahhs and Ehhs.
@4:33