When I see people pick tristana right now, I cringe

In 5.5 she just got a Q buff (-1s CD per AA on target with E). She's super strong if she can get the attacks in, just a little weak if not. It's a tradeoff.

Exactly what is wrong with Tristana?

I'm not saying she's strong, but she's a unique marksman. Her W provides more distance clearance than Ezreal/Graves/Lucien's dashes/blinks. Her Q AS boost and E bombs help her take down turrets faster than any other marksman (there are others who are fast as well, but especially late game Tristana wrecks turrets). Her ultimate provides further self peel.

Even when she's undertuned, if your team has little-to-no peel for the adc she's a solid pick. There's also nothing wrong with playing an underpowered champion. That just makes it more likely they main that champion. As long as your whole team is picking really off-meta you'll be fine with a few odd picks. It's a lot better to have someone with a champion they're comfortable with than someone on a champion they've never played before.