I think that crit builds have some pretty insane weaknesses.
First off, the standard crit build for most ADCs right now is B.F. Sword -> a Zeal item -> Infinity Edge -> a second Zeal item. For most ADCs, this leaves them with around 150 AD, no armor penetration, and no lifesteal until after third item, which means that they have no sustain and no way to break through anyone building some armor. The triple crit build is good at killing squishies, but leave you dead in the water if the enemy team has anyone who can build an armor item or 2 early. Also, buying a Thornmail on a tank will force ADCs to switch up their builds for lifesteal, MR, and armor pen, since they'll be killing themselves on the Thornmail.
Second, the 100% crit build is only viable on a few champions, namely Jhin, though I've heard that it can work decently well on Vayne too. For this, you'll as normal go BF Zeal IE Zeal, but then get an Essence Reaver afterwards. This leaves you with 1 slot to fill with either lifesteal or armor pen, and there isn't always a clear choice. Again, good at killing squishies, but bad at killing tanks and anyone who buys a Thornmail.
Third, a 50% crit build, IE + a Zeal item, doesn't let you have reliable enough crits for killing squishies, but lets you bypass some of the problems from the previous builds.
The last ADC build out there is a highly situational On-Hit build, where you pick up a BoRK. You'll do this when you need to kite juggernauts or get through massive health-stacking tanks, but if they return with armor + Thornmail, you're screwed. At the same time, you can't kill squishy champions particularly fast because you opted for % current health damage for shredding tanks.
The other problem for ADCs is assassins. A number of them can pretty easily dive the backline and kill you, meanwhile a tank is disrupting the frontline and ensuring you're unprotected.
Overall, I think that the 6 item limit and the drawbacks to doing triple crit/100% crit builds can overshadow the positives if the enemy team has a strong, armor stacking tank. Pick Rammus or something and rush Randuin's/Frozen Heart into Thornmail, you'll see what I mean. Any ADC who tries to kill you dies, while any ADC who tries to run past you to kill your team gets taunted. The bigger problem for the game's health right now isn't the crit builds, but how powerful the picks are. Twitch and Vayne have been heavily buffed by Riot recently without proper compensation, making them a lot stronger than where they should be. And, next patch, they're getting both nerfs. However, despite the fact that my main for several months now (Twitch) is getting nerfed next patch, I'm excited for the Kalista and Fervor of Battle buffs, as she is one of my favorite ADCs and in the state she's in I can't play her for some fun.