They lose acidity, which alters the flavor of the grape and pushes the winemaker into adding acids to help the finished product find some kind of balance.. But that balance can be hard to find because alongside losing the acidity, over-ripe grapes also contain more sugars which ends in a higher alcohol level..
High alcohol, low acidity - and the winemaker strives for balance.
Not a very helpful start.
Then theres the actual flavor differences, even after balancing
It's just not the same