Xayah exists?
But yea, let's focus on the recently nerfed Tristana with her 10% pick rate and 48% win rate who is absolutely mediocre until she hits 3 items.
Shorter range No mobility Takes three items to get to ie meaning she scales slower
Xayah is hard to all in but easy to punish for misplays.
What that means is that her ultimate makes her difficult to assassinate. Zed Ring her means she just uses it and backs off while she receives peel, effectively canceling it.
Her ulti does not, however, get her out of shitty situations she puts herself in. If caught alone in the river / enemy jungle by people, she needs to either be ahead and capable of killing them (fine), or else blow flash / have help coming in order to save her ass; her ultimate buys her time, it does not get her out of the wrong spot.
Trist is different. Her ultimate functions in much the same way; use it to buy time / get that assassin off of you.. But then she also has W. Which means that she can effectively worm her way out of most bad situations without any assistance, or summoners, needing to be blown.
The fact that her E is so fucking easy to reset - meaning she can assassinate people / is rewarded with it for simply auto attacking people - is insult to injury.
Counters to Xayah: 
She simply can't get close enough to them to trade back early, and late they can all in her and kill her very quickly.
tbh people hate champions based on their kit rather than win rate; just take a look at 
just because 1 adc is strong does not mean another isn't strong as well.
its funny because trist got knocked down a tier or two. xayah is the #1 pick/ban ADC as it is
Tristana still has much larger range, a gap closer with a reset, huge AoE burst from her bomb, and incredible self-peel on her ult.
I don't find either a problem tbh but one being good doesn't mean another isn't/can't be.
Tristana was nerfed, that’s why Xayah is the best now.
People are still moaning about Trist? I've heard more complaints about Xayah recently than anyone else.
