How to fix ADCs
Take away the critical stat and make their damage ramp up over a short time.
Crit is a bad stat. Not only because it's an unneeded form of RNG but because it is too easy to abuse the potential to deal BURST damage as an ADC. This is running rampant with ADCs as they can often out damage assassins that jump on them with two auto attacks. Keep in mind, most ADCs use attack move too so they respond with attacking the assassin instantly. This is terrible in design and makes ADCs way too hard to balance because a single crit can wildly throw off the balance of a fight. That isn't to say that crits can't have a place but it needs to be balanced around consistency and not a % chance to happen.
All in all, crit on items should be replaced with some form of damage ramp over time so that assassins have a window where they "assassinate" the enemy before they can react properly.
A good example:
**Accuracy +X%: **AAs gain up to +X% bonus damage to an enemy over 2 seconds while having vision of that enemy. (Ranged only)
Why accuracy as an example? Because it fits thematically and mechanically. The bonus % damage should bring damage up on par with buying crit (probably even higher as this might be a pretty big nerf). This serves two functions as a new stat. It gives assassins a chance to assassinate the ADC and not get out damage because the ADC got a crit hit for 50% of their health. It fits thematically as ranged attackers need to aim to hit properly and if they are surprised they will not hit as well. It allows counterplay and mechanically makes it more engaging as a stat and places more importance on controlling vision of bushes. They can still be tank killers and they will scale properly instead of getting "Lucky" with multiple crits they consistently deal better damage.
There could be other ideas too but I think the key point is to make a small delay in the massive damage ADCs can do. ADCs should not be dealing the crazy burst damage they are now. At least not in the first 0.1 seconds of a fight