It's because Maw/Hexdrinker are amazing in both offensive stats while making it practically impossible to be bursted down with one rotation - something an ADC should never be safe against. The shield automatically triggers without any required user input. Also, in the case of Maw, it grants additional AD, spell vamp, and lifesteal for no reason. Which means they could lifesteal enough during that shield period to stay alive. Which they should not be able to do. They should just die.
ADCs don't need armor.
Merc Scim is mostly fine, although the MR could be toned down slightly. The active still requires user input though, and the speed boost means it can also be used as a Ghost instead of CC cleanse. It requires good decision-making, hence, why it is more balanced. Also, just cleansing CC doesn't automatically mean they will win. They still have to play smart afterwards.
The cooldown on Edge is a little too low, but the concept is good. It requires a channel any damage can interrupt before you can gain a 1-time spellshield. Currently, if damage interrupts it though, it places it on like a 4-second CD before letting you use it again. I think it should just count it as used-up and put it on full CD though, in that case, to punish for using it at a bad time. Also the cooldown should be at least like 90 seconds minimum.
ADCs are never sentenced to a quick death out of hand. They are one of the strongest classes in all of League, and one of the only ones to get continual favoritism from Riot. If an assassin kills them, the ADC player did something bad. The assassin was not OP. If the ADC does not know how to kite and position properly, along with warding, playing smart/safe, and working with their team, that is their fault and they deserve to be deleted. It is understandable if the rest of their team failed to peel correctly, but that is the general gist of it.
The only assassins that are even slightly strong at the moment are Rengar and Kha'Zix, and while Rengar is the stronger of the two, Kha'Zix has the better single-target killing ability thanks to Invisi over Camo. However, no one will be alone unless they are stupid, so Rengar is better because he can actually fight, cleanse CC, use hard CC, and heal. However, Camo means he can be easily countered and counterplayed and stopped from killing the ADC or mage or what have you.