if just mages could build them without totally giving up their damage....
Adcs and Ga is the same story if not far far more:
/ 
instead of 
/
is not that bad of a trade considering you can have lategame more surviability and the penetration of 
, 
and/or 
. Cdr also in other items and runes in it.
instead of 
/ 
/ 
/ 
/ 
/ 
/ 
/ 3rd 
item is what?: You loose at least 10ad(
items) plus 35% bonusarmorpen and gw/healthdamage. Or you loose 40ad and the effects you nicely described below for dd and bt. Or you loose 30ad, 20% crit, 30%cdr, manapassive on er. Or you loose 30% crit, 30-45% as and 5%ms at least. Even compared to maw/mercurial (here you could speak on strong effects) you loose 10/25/30(with maw passive) ad and trade some mr for 30 armor that, to be fair, tabbis could give you also plus the tabbipassive and just a loose the 35%as on 
(if you realy,r ealy need armor).
Compare that to 
that, again, gives you 70ap, 10%cdr and 45 armor (more as tabbis). THATS not soo bad.
GA it's also cheaper,and its passive is far more stronger than zhonya's one (and in this case it can actually save you from an AA,zhonya/stopwatch means delaying your death by 2.5 sec)
I already pointed out that no adc beside maybe 
builds ga. Even if it costs less the stats are not for adcs, ga is far better for bruisers or ad assassins. Same time every amge if he plays vs a Zed or Talon can easily go Zhonyas. He can even have the rune Perfect Timing to have a free 
and Zhonyas costs them 250 Gold less. Or go magical footwear and free boots, 300 Gold not spend which is availabel for other items like Zhonyas as well.
If an adc still runs perfect timing or has run it in the past he never insists to build a cheaper, better ga. He uses it for a saver laning phase and than sell it again.
: 30% damage reduction (you can easily sustain it back if it's as DPS,which against a burst champion means denying this damage),on 1800HP means it can block 540 damage alone,not to mention its lifesteal
: 350 HP shield + 20% lifesteal
: 12% damage reduction (on 1800 hp 216hp as damage reduction)
AP items : 300HP
Also why do you point out that ga is cheaper as Zhonyas (i could repeat myself that no adc buys this) and then act as these items are free? Beside pd (which only redceds the damage of the person you last attacked: 1 out of 5, you have to hit him, an invis talon or you cc makes this hard) every Ad defense item is more expensive as every ap defense or health item. DD and BT are literaly some of the most expensive items. You can argue that Zealitems are to strong but Mages get their Defense so much cheaper.
And Banshees/zhonyas not only give 70 Ap and 45arm/60mr, they also give 10% cdr and that s a direct damage and utility increase too: You can spam your abilitys more. Even on full cdr transcendence would give 20 additional ap so you would even be higher as most ap items. Ga gives crapy ad, only 30 armor and no crit or as to boost additional damage. Mercurial only gives 35mr and no as or crit to boost damage. Wits end only gives as but no ad, 40 mr and just gets effective in longer fights while also is far more niche as zhonyas (but kinda even more common as ga on adcs). And maw has 45mr but its real effectivness just starts if the shield plobs to burst magic damage so it gives you more ad and some ls/sv well.
considering those items give the same health if not more in shields/damage reduction,and yet they give offensive stats (like AD/AS/crit) like any other offensive item while allowing to sustain back the damage,while for AP you are giving up AP/CDR/mana and since ADs have less mandatory items,i wouldn't call it fair and idk how you can say AP items are better against burst
[zombie-brand-facepalm] [zombie-brand-mindblown]
You want me to give a list of stat tradeoffs? ok fine, here on mages and adcs the 10 most used defensive items:
: 70Ap, Cdr (offensive), Armor (defensive), stasis (both, can be used to wait for mana/cd)
: 70Ap, Cdr (offensive), Magic Resistance, passive (defensive)
: Mana, Cdr, Aura (offensive), Health, Magic Resistance (defensive), Eternity (both)
: 100Ap, Mana (offensive), Health (defensive), Eternity (both)
: 80Ap, Madness, Tornment (offensive), Health (defensive)
: 80Ap, Pen, Gw (offensive), Health (defensive)
: 85Ap (offensive), Health (defensive), Slow (both, especialy strong with Liandrys)
: 60Ap, Cdr (offensive), Health (defensive), Fire Bolt (both)
: 80Ap, 40Ad (offensive), Omniheal (defensive), Ligthning Bolt (both)
: 50Ap, Cdr, Mana, Awe (offensive), Mana Shield (defensive)
Average: 3 offensive tradeoffs per item, 1.8 defensive tradeoffs. Average cost 2930 Gold
: 40Ad (offensive), Armor, Passive (defensive)
: 50/70Ad, Cdr (offensive), Magic Resistance, Lifeline, Ls&Sp (defensive)
: 65Ad (offensive), Ls (defensive), Qss (both, especialy with the ms)
: 42onhit, As (offensive), Magic Resistance (defensive), passive (both)
: 40Ad, As, 8%onhit (offensive), Ls (defensive), active (both)
: 80Ad, Cdr (offensive), 2 passives (defensive)
: 80Ad (offensive), Ls, passive (defensive)
: Crit, As (offensive), Lament (defensive), Spectral walz&Ms (both)
: 55Ad, Lethalaty (offensive), Health (defensive), active (both)
: 30Ad (offensive), Health (defensive), Slow (both)
Average: 2.3 offensive tradeoffs per item, 2.1 defensive tradeoffs. Average cost 3105 Gold
So Ap items always give min 2, max 4 offensive stats/abilitys and min 1, max 3 defensive stats/effects while Ad items give min 1, max 3 offensive stats and 2 defensive stats/passives, 3 on Maw.
BUT: Adcs values Ad/As/Crit high. Most items only give one of them. If they give some other offesnive stat like cdr than that is fine but has nearly the same value as the 40 Ad on Gunblade for a Lb.
Also while Ap have not so much ways to get Heahlthsustain, Adcs have not so much ways to get Health.
Adcs have maybe to much strengths like not beeing so dependent to mana or cds, zeal ms, the ability to sustain back. But at the same time they have also weaknesses like small health pools and that Gw, CC and Burst makes most times the sustain not so effective.
The REALY BIG DIFFRENCE lies in Supports. Adcs tend tos tay around them and they give them additional shields, heals, damage reduction, health (hello lulu), cc enemies, even ad, as, onhit which profits just adcs more.
I would say that adc itemization is mostly (not enitrely, zealitems like pd are defiently a problem) fair in the direct 1v1, but adcs are never realy alone and supports have such an high impact to not only add some power but realy multiply the power of an adc while mages just have their items for the main part.
plus electrocute or Dark harvest will obliterate a mage, but an ADC has some hope buy building 
has a massive cooldown and has to be fully stacked.
, 
exists as Alternative if you have 
in your team.
]
)
in my ranked games for some freelo!
because mages are in an inherently advantageous position with their spell range. theyre able to poke and apply damage to the assassin much more repeatedly than the assassin can deal back at them, while they can potentially burst their opponent in a similar fashion.
