An assassin role, by default, is always going to struggle vs more coordination. Its why you dont see shit like Spies in competitive TF2, while they can completely dump on random pub games. Assassins punish mistakes and bad coordination. Its just at one point in League's existence, Assassins were able to do more than that, and all people can think about is the days gone past.
We have all been in solo Q games where people dont have the right champs, the right comps, or even the right coordination to punish assassins, and in those games, you have Zeds 10-0, Fizz's 9-0, etc. And then you have another game with a support Lulu doing her job, strong coordination, and its not as easy as someone just clicking R to win like it was in that previous Brand support game where that Ashe had zero peel.
If a team is playing coordinated enough to stop you, you have to be coordinated enough of a team to get around it. Wait on them to use their tools on someone else before you go in. Most of these people whining just wanna yolo dive 1v3, and then they get hit by every single CC/shield known to man, and complain about how imbalanced the game is. How imbalanced would the game be if you were able to yolo dive 1v3 and kill someone regardless of what their team did? It would be FAR worse. If you can't wait on your team to force some CDs out, then who is to blame. I mean even that OP ADC you are complaining about sometimes just has to dance around in the back waiting on the malph ult or whatever CD they want to avoid before they can realistically get into the fight. What makes your role any different.