Assassins will always feel frustrating to play against.

Assassins are meant to do 1 think really. Delete high priority targets and (hopefully) get out alive. And while they have a high risk high reward play style, meaning you have to get fed on them to do well, it's not really a play style that makes the game enjoyable. Either talon pops off and deletes you with seemingly no counter play and goes 10/0 or he falls behind and compared to a control mage he becomes a lot less useful at later stages of the game. Now the assassin rework was meant to solve this issue. It was intended to give the assassins an identity giving them something extra, reduce some of their burst and add delays and combos for counterplay but it didn't quite go far enough in that directly.

I think the concept of what an Assassin should be needs to changed to make them feel less frustrating. The risk and reward should be placed into their kit rather than requiring them getting fed off bot lane. Their kit should require high risk high reward combos, they need to pull off to do additional damage using the bar of burst mages as a comparision. You do a combo sucessfully, higher than a burst mage. You mess it up less than one. On average you do slightly more damage than a mage, but provide no utility or cc, and your ultimate should require set up to pull off or conditioned like a yasuo ult rather than point and kill or press R.

You may hate hearing Hasagi... you may also hate his infinite dashes, but if you died to his combo, cause he landed a SKILshot which knocked you up, well it's pretty much your mistake and you deserve to die. If another fed assassin just needs to press R to kill you, well not much you can do in that case. TL;DR.... Make assassins Damage conditioned on landing combos (even more so), make more of their abilities and ultimates skillshots, but do higher damage than burst mages in general and they'll feel more fair to play against.IF playing an assassins felt like you were opting into playing a mobile burst mages doing slightly more damage but providing no cc or utility, they would feel more fair to play against.