tank itemization is not in a bad spot, thing is that ADCs wait out until the enemy tank builds a defensive stat and then counter it, for example if I'm playing Cait and I see that the enemy has a warmog's I consider getting a botrk even though it's not as efficient on Cait herself, if the enemy has heavy armor, then I just get a vampiric scepter and try to get armor pen as soon as possible.
it's also truth that most tanks (at least from gold to bronze) don't try to counter build at all, if you have a warmog's and I have a botrk, you should probably get Randuin's, instead of a Liandry's... also, remember that armor alone or HP alone are only half effective, but using both will increase their effectiveness ten fold, in that regard:
is an awesome rushing item because it gives both armor, HP and Damage so ADCs will need to foresee if you'll go for more HP or more Armor, if they mess up and start building something before you do, then you flank their build with something else, say if they build BoTRK, instead of going for Warmog's, you go for Randuin's right away
specially is the greatest defensive item since it lowers crit damage, lowers attack speed, lowers movement speed and has a TONS of armor
is particularly good too but you gotta weight down if the enemy has too much slows or not since those WILL lower your effectiveness so you'd be better off with 
In contrast though,
scales off of bonus armor, so rushing a thornmail is never good, but if you have a
and
and the adc is still a problem then you should get thornmail and the dmg reflected will be seriously high. Adding grievous wounds to thornmail would probably break the item, but I do believe that Grievous wounds SHOULD be a tank stat as opposed to a dueling stat (only
and
has them).
ADCs don't have armor pen built in their kits so in reality armor will shut them down pretty good, unlike
who will laugh at any defensive stat but whose position is always off so they are susceptible to CC
Edit: I forgot to talk about magic resist, magic resist itemization is the one that's most off currently but the good news is that ALL defensive magic resist Items comes with HP so they are pretty effective on their own. If the problem isn't the ADC but an APC, then your priority should be an item that protects you from magic damage AND physical damage
is the one that comes to mind, since the shield will protect you from poking and they'll need to burn a spell to pop it.
has both Armor and Magic resist, so it kinda works, plus pushing power!
if you have a healer as a support you are effectively doubling your and his effectiveness