The AD defensive itemization for marksman is limited, at best.
The absolute BEST item for defensive items are Edge of Night, and cry as you will, it isn't very strong defensively, and Mercurial Scimitar, which, given, is a very strong item.
We can also buy Banshee's Veil, which means trading in a Bloodthirster or an item of around equal value, or a Guardian Angel, which we would also have to sell a Bloodthirster for.
With the Warlord's Bloodlust keystone falling out of favor, lifesteal is becoming much more important to build, and the defensive itemization for AD carries becomes even more invasive into our builds.
When you compare AD carry defensive items to AP carry defensive items, it's a total landslide.
AP carries get Abyssal Scepter, Zhonya's Hourglass (given, it's is in a pretty bad spot right now), Archangel's Staff (if applicable,) Rylai's Crystal Scepter, and on top of all that, you can STILL choose to buy a Guardian Angel if need be, because all of your defensive items still give enough AP to be relevant.
The defensive itemization for AD assassins is more comparable to AP carries, but even then, assassins really only have Edge of Night, Maw of Malmortius, Mercural Scimitar, and Guardian Angel. While, in my opinion, the power level of these items are quite strong, having all of these items would force out Youmuu's Ghostblade, Duskblade of Draktharr, Last Whisper, or Black Cleaver, ALL of which are incredibly necessary in the majority of games.
I think that the assassin defensive itemization is actually quite balanced, it's just that the lethality of items are incredibly abusable on a lot of champions, making them deal too much damage for what they're paying for.
The place to nerf assassins isn't in their defensive itemization, it's in their gold efficient build path. They need to be nerfed mid game, as generally assassins are quite bad in the laning phase, and perhaps they might get a compensation buff early or late game. Of course, it would all be speculation based on how assassins get their mid game nerfed, but the point stands that AD vs. AP defensive itemization shouldn't be a reason to be hating on AD champions.