Tristana has always been a problematic champion design, and her first gameplay update addressed no one of her problems.
She's too safe. As an assassin or fighter, there's no way you can kill a decent Tristana even if you're 1v1, unless she misplays or you outplay her really hard (or she has everything on CD, but then she's not that good).
The longest base range of any adc late game, AoE damage, waveclear, a jump which resets on takedowns or max stacks-E detonation, a 1000 units knockback that also deals some good damage, and one of the best turrets-melting tools.
I... just can't find a weakness in her kit. She has everything an ADC would ever want and she's either so bad to be unplayable or viable and permapick, based on how good crit itemization is (and right now it's broken and grossly underpriced).