I don't think she's that strong, but that isn't really the issue that I see debated about Trist.
There's a huge difference between strength and game health. Those two concepts are not the same at all, and I bring that up because the issue with Trist is her design, not her strength.
What's Tristana's weakness, would you say? She's a late-game hyper-carry with self-peel, excellent siege tools, good range, good mobility with a reset, and the best attack speed steroid in the game. The only thing that I think she really lacks is poke, and you can even argue that she has a small amount of that with the way her E works, at least in lane.
The issue is that when the meta shifts and she becomes strong, there is no place to punish her. It's often true that champs become meta when the meta covers their weakness decently (see Vayne recently when lane sustain was too damn high), but with Trist, it feels like she just gets randomly strong when other ADCs aren't. She just isn't a very interesting opponent, and I think that's the big issue.