AP Versus AD
There's no single "the problem" on this, but with the identity of mages shifting to mid-range DPS (since baron nullified defensive waveclear), they're running into big survivability and viability problems. Lemme lay it out:
Flat Armor Seal Nerfs: only a true veteran remembers knocking them from 10 to 9 for being "best in-slot". Am I saying 1 point matters? No. But this was a symptom of how necessary that armor was. What alternatives did you have? 3 mana regen per 5?
Base Stat Nerfs: the conversion from base and per-level stats to Base and Growth stats means weaker early, stronger late. This mostly hit HP. That specifically hurt APs the most because everything they need comes from items. By the time their hp gets high enough to be good, people have items. And APs lack the resistances to make the health worth anything anyway.
Sustain Is Virtually Nonexistent: Only one or two APs can afford to ditch a slot for
. Even then, it's too limited. If an "aoe" spell hits only a single target, you get 5%. That's not eveb as good as 800 gold for
and you **still have to burn mana to even use it **. This puts them way behind any AD for sustained DPS, and that's the direction all recent mages are going. The only solution is completely broken sustain like Ekko, Ahri, Cass and then it ends up getting nerfed anyway for being too much for free.
Item Changes Block Needed Stats: it's good that APs can get defensive stats to help bypass the horrid lack of sustain, but it's fallen way out of wack. But
and
have both lost significant HP. While some went to
forcing a champ to buy both isn't viable for most because you already have two must-buy items in
and
(some people even consider
must-buy). This is without even approaching the topics of mana, mana regen and CDR. So in this case, you'd have 700 empty health backed by no resistances, no extra mana or regen, no CDR AND you're already 4 items deep.
Mag Res Is Far More Abundant : Zhonyas is now 45 armor. It's also the only damage/armor item usable for a non-tank. That leaves you with a max 53 with runes.
alone beats that out, and there's still MR glyphs and
yet. It's almost double the stats.
The Final (and Biggest) Problem : This

What's that list? That's all the champs who with (often large) amounts of armor shred or ability to just ignore it completely. Now let's look at the same for MR...
Well, there's
and, uh...well there's
. And...dang, that's about all of them. Did you notice there's about as many champs who can buff allies' MR as there are ones who debuff it? And I'm not even getting into the fact that
can get applied in AoE and on physical DOTs
Clearly, you can see where we're at now: there's armor shred all over the place we nerfed survivability, then we nerfed the only armor item, now we just buffed the hell out of marksmen and AD bruisers. I won't even touch the % damage reduction on Phantom Dancer or Skirmisher's Saber. So the champs that have to manage a resource get minimal sustain, which necessitates defensive stats that are now minimal against the damage type that has a plethora of shred for it and you have to do it while half your item slots are dictated to you, but those 3 items (the shoes, hat and stick) don't give you any of those things you need