The problem with ap items is that they are too good.
They give too much ap for also giving other stats, like you can have certian mages like brand having almost 3k health on their "full ap glass cannon" build, then theres items like the tear items that are over 160% gold efficient when fully stacked (better than fucking crit items efficiency when combined together).
Then you add hourglass, then you add permaslows with rylais, then you add *ignore literally every mr item) void staff, then you add the best 1 item powerspike in the game that gives everything a mage could possibly want (ludens), and theres not a lot of counterplay left to ap items, hexdrinker is the only thing thats ever effective against mages/ap champs but thats only in the early game, as maw of malmortius gets outscaled horribly hard by the mage itemization.
I think some mage items need to be split into multiple other items (would also make room for ap assassin and ap bruiser focused items, which are very lacking atm as riot focused too hard on making perfect mage items), like an item with such utility like rylas, hourglass, or even seraphs should not be granting so much ap on top of it, while some other items like ludens/void/deathcap are just straight up overtuned and needs their numbers to go down, mages are also scaling too hard for how good they are in general at most phases of the game, like if you build crit (a very good scaling build) you're weaker earlier as a trade off, now if you're a mage you get to rush ludens and then get to scale anyway with deathcap/void, you get the best of both worlds.