If you're not AD against WW, Malzahar, Skarner, what now?

Quicksilver Sash does everything it's upgraded counterpart does, minus the speed boost. You knew what I meant but wanted to force in a mention to two other items that give AD. We all saw what you did there, and you should feel bad about it.

I don't know why you're trying to argue with me here, particularly since my first post had absolutely nothing to do with items. There are other people in this thread you can talk about items with who quite clearly already want to talk about it.

So please, go talk with the people who are already talking about items. I came here to answer the OP's question, not debate about which damage type has the better effects to their items.

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Roleplay start now : But, why ADCs are allowed to have this things, if you should be punished for bad positioning ?

Somebody probably has a better reasoning, but the way I see it..

ADCs's build path heavily rely on particular rush items. Rushing a QSS on an ADC isn't giving them the needed attack speed or AD to snowball, only what QSS builds into benefits the ADC. In a way, building a QSS on an ADC sets them back or waste 1300 gold when they could get a BF item instead. Its a teamfight item if you're the team's main source of damage and people can protect you.

With an AP mage, they can honestly build anything. Even if you build incorrectly vs. your matchup, you're still going to deal damage cause of how your kit works. Let's not look at vel, and look at veigar for example, he can protect himself with his cage, while still being safe under a tower and dealing tons of damage. Syndra as another example can push somebody away if they get too close, poke with Q, and just farm under tower. Now the way to kill these two, would be to lock them down with CC, say a WW ulti. Give them a QSS item, and now their only weakness is taken away, how do you kill them if they don't get caught out? How would you kill a syndra with QE, veigar with E, Vel'koz with E and rylais, etc if the moment you do catch them, you have no way to lock them down yourself.

That may sound a bit off directional, since I basically said "adcs are punishable build paths, while APs have tools to kite" but if you compare kits, you can see why a QSS would be more "balanced" to deal with on one champ over the other:

vs.

Lets say you have a jinx with QSS and an annie with QSS, either one in bot lane split pushing, and your is on her way to pick them off. Jinx's only counterplay to stop elise from securing a kill on her, is to land an E, which is kinda tricky, or QSS the cocoon and run away. However she's not guaranteed to live as elise can still kill jinx with her abilities and chase her. What jinx can't kill with just autos she's going to die to.

Now annie, elise is ganking a splitpushing annie (kek), annie can QSS the cocoon, but that's not done.. annie can then either insta shot elise, land a stun and walk away while jinx can't, and has maybe bonus bulk due to RoA+Abyssal+QSS.

This is where QSS can be balanced on an ADC over a mage, a mage has the tools already if played right, and their moment of weakness is getting caught out. If elise cocoon'd annie she could burst combo her before annie does the same, but because of QSS, annie can burst combo elise with a counter stun before she does. With jinx, jinx can have it as a safety measure or to stop a stun in a TF, however she's not going to turn a 1v1 completely around with QSS, that may have set her back by 1300 gold vs. the other ADC making her actual laning punishable. An ADC doesn't have "burst" to completely reverse the trade vs. something with CC, a mage can.

I don't mean for this to sound like "so QSS currently is stupid on annie/anything on paper?" as it doesn't since the stats it gives aren't significant, the burst would be gone making the math kinda lame, but pair QSS with CDR+AP/M.pen and it can be an effective core item on a lot of mages.