I feel like it shouldn't be 'easy' for an assasin to kill an ADC. In a 1v1 fight between similarly geared adc and assasin, the assasin should have a signficant advantage. But the ADC should have at least a fighting chance and an opportunity to outplay. In a teamfight though, you can't be expecting the assasin to easily dive and kill the enemy ADC every time, 100-0, because that is just busted. But you should have a really good shot, and if the ADC makes a mistake, or the support doesn't peel, then the ADC should die, or at least be zoned out of the fight.
Besides, even if you are an assasin, it doesn't mean you have to work on your own completely. In a teamfight, your team/support can help the assassin access the target. For example, if I'm playing Eve going in to kill the enemy Vayne, I would likely fail if their Support Janna for example, ults. But if our Morgana gives me a black shield before I go in, I can jump on Vayne regardless, Janna can't ult me away and Vayne's condemn won't push me away into a wall.
The rest is just skill. A 3 item Talon should always be able to kill a 3 item Tristana with ignite, in a 1v1, where the Tris can't see you coming. If you correctly put in all of your skills the moment you jump on her, she should be dead by the time the casting animation on her jump or ult finishes. They aren't instant yknow.