What most people dont get about Marksmen and Assassins...
People get super mad after getting one shot by an assassin, but they dont realize that that is their job. Assassin champions ARE MADE to kill. If they dont kill, they are completely useless. They are made to burst, deal a shit ton of dmg in little time and then get out. They may feel super unfair, but that is what they were made for, to kill you. They excell in 1v1s. In mid lane, if youre getting killed by an assassin, just back off. Because you wont be able to kill him unless youre ahead or you are an assassin yourself. He will kill you before, unless you play carefully and whittle him down (this is for mages mainly), then you can kill him before he bursts you.
Marksmen... Ohhh marksmen, how many times have I seen a Jinx trying to chase down a support and then get destroyed by the whole team, and still dont get the kill. Marksmen are about positioning, they do NOT excell in 1v1, but in teamfights instead. Marksmen are incredibly strong WITH their team as long as they are well positioned. Since they deal sustained damage, a lot, and need peel from their allies. There are excpetions, Vayne is an amazing 1v1 champ, guess why... Her secondary role is Assassin. There you have an example of both things together. But does that mean that she will destoy everyone because she excells in both instances? Of course not, since every champion is balanced out of their strengths and give them some weaknesses. She is squishy, very, and is extremely vulnerable to cc.
Just a small, extremely random share of my opinion.

hahahahahahaahahahahahhahahahaha no(not whining here just saying she's good at 1 vs 1)