ADCs are hitting spikes later cause of armor pen change. The god tiers have earlier spikes than the others or they have uses beyond itemization that allow them to stall till they spike. Jhin and Cait have utility and range. The thing that separates them from another utility ADC like Ashe for example is they are stronger lane bullies than Ashe but that doesn't mean Ashe is bad her kit can be utilized greatly like shown at worlds to sway those match ups. Before preseason vayne was in a good spot her crit build relied on RNG but it still allowed her to duel once she hit her 2 item spike. It was fair because if she hit this spike while behind she had to outplay against pink wards and a more fed ADC to get kills which is why watching vaynes pop off is interesting. With the new patch where there are no pink wards she doesn't have to outplay nearly as hard or even at all just hit 2 items and tumble crit. Which is where the stealh in her ult becomes a problem the build itself is fair but the fact that she is taking less skill than before and has no real form of counterplay (not being able to be hit because of stealth negates a lot of damage she would take otherwise) means she can be useful and is not punished for playing poorly as long as she spikes. She's basically rewarded for playing worse cause of the stealth. Out of all the ADCs she is probably the strongest in combat by far at 2 items damage wise. Utility ADCs at 2 items can use their utility to impact a team fight but would deal less damage themselves than a vayne would at 2 items. Other ADCs that are participating in combat are much more easily punished compared to vayne even if they are playing better than the vayne.