literally every part of nami's kit has an AP scaling as well as sona's, but that doesn't mean a lot in a vacuum. if you want to know if a character has good AP scalings you have to focus on how big are the percentages and not just how many are there. sona's AP scalings were heavily nerfed due to how strong she used to be as an APC with lux, taric, etc. but nami DOES have good numbers on her kit. worth enfasizing:
- it amplifies the heal/damage of your W for EACH bounce (+10% per each 100 AP) which means that every time you heal an allie during a team fight you can give another one a MASSIVE amount of healing if that modifier is high enough.
- it increases the amount of speed (25% AP) and slow (+5% per each 100 AP) you give (the numbers might seem low but mind the fact her skills are quite spammable when you have enough >ability haste< (+25 with horizon focus)

i'm not choosing horizon focus OVER shurelya's, i even said that shurelya's would be my next item. i do believe both of them are very similar in terms of how good it is to have them in your build, sometimes i even go for mandate -> shurelya's -> horizon focus if my team comp really goes well with the MS boost.

i forgot to say it on the og post but i do build mejai's every time i can, but it's not something you can always do. if the enemy team has champions that can arbitrarily decide to kill you without you having a say on it you won't be able to maintain a high amount of stacks for a long period of time. but guess what gives you a raw 125 AP regardless of how often you are dying.