AP Scaling Utility is a bad stat, Sona being the worst offender of this.

Our 3 champions with AP Scaling Utility are

Nami has scaling AP utility with her passive, which isn't a huge part of her kit. Her bases are pretty decent, and aren't affected by AP, which allows her to be pretty good in the support role.

Sona and Lulu have AP Scaling utility across of all or nearly all of their spells. So here's the problem with both of them

1. Their base values are rather low in the realm of supports, which means they need to scale... which means they can't really support real well early on.
2. AP items are now really expensive after the cost adjustments in patch 5.22 (exceptions to Lich Bane, Abyssal, RoA, and Nashor's) , and supports in general aren't given a lot of gold for their budgets.

So, Lulu has been shifted around to top lane where she can bully melees, and midlane where she can turn assassins into squirrels. She gets a bigger power budget, and now scales really well in the fact that she's a pretty good replacement for Orianna in terms of utility.

So, what about Sona?

She's too squishy and doesn't have enough peeling tools for a solo lane, and she sort of needs another person in order for her kit to work to its full potential or else her heal and damage is pretty much irrelevant in midlane, and her waveclear is lackluster in comparison to the Yordle. Her ult is OK, but there are other midlaners who have spells that can imitate Flash- Crescendo with no flash such as . All 3 of which definetly have a form of waveclear.

In the support role, she needs to buy AP stat after finishing FQC + SS, as her utility scales off of her own AP. She normally builds around 4 AP items a game . And still does a great deal less damage in a given game than other supports that build 4 AP items , and even does comparable damage in an average game to who builds 2 damage items. Let's not forget that she deals no form of hard cc without being at least Level 6 while the other 5 have some form of hard crowd control.

So in short, AP scaling utility is bad in a support role is bad, but Nami still has good bases, and Lulu can survive in a solo lane. But Sona isn't really accomplishing that much in her current state except somewhat average damage and a small amount of sustain and she should be reworked, by a support main this time hopefully.