Let's take Vayne for example.
It's Vayne top lane vs you, a Darius, for example (before somebody comments, yes I've been playing Darius recently and feeding, but I never went against Vayne top. This is an example). You sit under your tower for 10-15 minutes before you finally get out of lane. All throughout lane you get poked by Vayne if you try to farm. If you ever try to engage, she uses her Condemn and Tumbles back, so you can practically never engage on her, just want to cut your own dick off because of all the poke you're taking. You buy tabis and a Bramble Vest, but she still cuts through you because of her Press the Attack and Silver Bolts.
How about with Tristana, in her regular lane, bot lane?
It's Twitch/Janna vs Tristana/Blitzcrank. Every time your Janna tries to engage on Tristana, she uses her W, and Blitzcrank follows up by hooking you or Janna. When your jungler finally ganks, Tristana ults the jungler, and W's away. At least you can 1 shot her in the lategame, right?
Every time you destealth on the team when they're clumped up, Tristana just ults you away.
This is not fun game design. Somebody meant to do damage over time shouldn't have an incredibly safe kit. Somebody whos main drawback should be being squishy and have no gapclosers shouldn't have multiple way to get rapists off them. This is why people complain about Vayne and Tristana, because they pump out insane burst, yes, BURST, while also being some of the safest champions in the game.
But what do I know. I'm just a Bronzie, right?
Edit: By Janna engaging, I mean using W+Q to hopefully catch somebody out. Or even doing a hype Flash + Ult combo like an Insec.