Riot, for oh, I don't know, about 4 years now, probably longer, ADC damage has been a major complaint by the community. Many people point to Crit's RNG basis as the big issue, as it's "Imbalanced" and "Unfair". However, Crit itself is not problematic. In fact, it can be used to create unique and powerful designs for champions to be built around. Tryndamere, Yasuo, and Ashe all have unique crit interactions (Double Crit chance, Increased Crit chance the longer battle lasts, and More potent slows) that make them "Unique".
However, I'm going to be pulling up Dota's Design choices for how Crit is balanced in Dota 2.
1. Crit doesn't stack. Basically, Crit damage, and Crit chance are independent values that don't stack across abilities or items in the game. This opens up item diversity among champions that have crit built in by changing what items they buy since they don't waste slots on attempting to stack up to 70-80%.
2. Crit items don't have Attack Speed on them. In fact, there's actually only 1 "Crit" item. And it's stats are Raw Damage, Crit chance, and Crit damage modification. (Essentialy Infinity Edge).
3. Crit uses "PRNG" or Pseudo Random Number Generation. This ensures that you'll never go too long without getting a crit, because your odds to crit actually go up the longer you go without a crit.
Going over these a bit more in depth.
1. By having crit chance not stacking, Ice Frog and Valve can better tune champions based on the type of crit they have access to, whether it's from a passive, or if the crit item is something they'd prefer. This means they can adjust base stats to better open up their item builds to allow them more freedom in situational items since they're less reliant on stacking up enough crit to amplify their damage.
2. Crit chance and Attack Speed actually stack exponentially with each other by the nature of how they interact. The more attacks per second you can do, the more likely it is that you land a crit, and if you have increased crit chance on top of it, then you increase the likely hood of a crit yet again, this creates a powerful loop that makes Crit Chance/AS items Hyper efficient in terms of builds. This is why you see the 80% crit chance ADC's as the "Superior" adc's in most cases, because the increased AS from 2 zeal items is far more efficient to damage than having both IE/ER and a zeal item. yes, the low cost of Zeal item upgrades does factor into this, but it's more the mathematical superiority of the Exponential increase in damage from double zeal items. This is actually the biggest reason why people have complained about ADC's. The complaints about damage, really exploded around the time you changed zeal upgrades to have multiple choices outside of Phantom Dancer and Trinity force. By adding in Statik Shiv, Runnan's and then eventually Rapid Fire canon, you ended up with multiple efficient crit items that amplified ADC damage by magnitudes more than appears on the surface.
3. Using PRNG vs RNG. I don't know if LoL actually uses PRNG, but if you don't then it makes sense why you'd allow Crit chance to stack, as simply increasing raw RNG % ensures that players won't ever go too long without some critical hits. However if you do both. It makes 70% crit chance and 100% crit chance almost impossible to discern the difference between.
Overall, I know this is a long post, and most people will discredit me because I'm a filthy gold support main, but I've also played the game since before season 1. (I have Judgement Kayle to at least demonstrate partially my duration in playing). And have seen every single change Since Caitlyn's release (She was the first champion I remember being released when I started playing. Renekton came right after her.)
Zeal's issues have 3 things, 2 are tied to notes above, and the 3rd is pretty obvious.
1. AS+Crit Chance is far too damage efficient.
2. Crit Chance stacks, reducing item build diversity for most ADCs
3. Zeal Items are just too gold efficient. While most other classes must spend around 3k to get key items, ADCs generally only spend about 2.7k for their items, meaning that by their innate access to higher gold income and safety, they are more likely to get ahead, and stay ahead and dominate games.
Remove Crit Chance's Ability to stack. Make Crit based ADC's have the ability to choose between IE for big bursty crits, or something like Runnan's for weaker, AoE crit damage. Then make all other Crit items into new items, or rework their stats and build paths to open up their diversity to more champions. Rework Champions that rely on larger crit values to have passives that increase their crit chance passively if necessary (Like Tryndamere and Yasuo).
Zeal items are obscenely efficient in both Cost and Damage amplification. Crit Chance Items should be reworked to provide either Crit Chance or AS but not both, and ADC's reworked with the new items in mind.