At some point she got better at singing. Now, I don't mean she was ever bad, just that older songs like Holiday, have a more amateurish singing style than something off Confessions on a Dancefloor for example.
I’m no expert but I feel like she changed up her style to suit emerging trends with each record while also enlisting the help of REALLY good producers and sound mixers. I found myself listening to some songs multiple times today from Hard Candy. Although I think her best album is Ray of Light.
I've been going through Yes's divisive (to prog fans) late 70s-80s catalogue, even Big Generator which I'm only finally listening to for the first time after 15 years of being a fan.
Say what you will about Yes's turn to pop, and the beginning of "over-produced" music, that world class production by Trevor Horn which defined the 80s is what elevated Yes over other contemporary prog or ex-prog acts IMO (Genesis, and even, I'm sorry to say, Rush).
And I don't mean because it made their sound catchy and radio friendly, but because it brought out the unquestionably outstanding skills of the band, and the tracks would've sounded sick even if it was a rehash of a 2 hour long 70s prog brick. Slick production isn't always the commercially bland conformist evil thing some people make it out to be...