ADC are NOT supposed to be strong at all stages of the game.
Due to the low end cost of support items the usual time frame in the game where ADC was most vulnerable isn't there anymore they have no time of weakness throughout the match they just scale harder and harder. The time mark i am talking about is the 12 thru 15 min mark where most people have there first item this time mark SHOULD be the point in the match where an ADC can be punished the most before they begin to finish there items.
ADC shares exp bot lane yet is outperforming all other classes at a time mark where they shouldn't
This is from adding unnecessary power to them over a set of patches specifically in my opinion:
-
The mid season update where all champions that gained no MR per level suddenly do including ADC.
-
The exp catch up change. Not sure which patch this is was in.. but this was implemented to fight snowballs from over occurring im assuming. Making throws more frequent. This ALSO plays a part since ADC would always be behind in level due to sharing with the support i believe that this implementation is allowing ADC to snowball harder since the system behind leveling is constantly reevaluating that the ADC is "behind" in exp so they are slightly more ahead in level than they actually should be assuming the ADC is getting bounty kills.
and
- Buffing
early for no reason.
An 
with 6 deaths and 0 kills should stay behind the curve. But the exp catch up allows in this example for Ashe to simply land one auto and kill an high bounty target and suddenly have the relevance of a near mid game scaling 
when paired with a support. As i explained in a previous thread support items are too cheap. All this combined and ADC is the strongest class at near all stages of the game.
**
When the ADC is at a weak point, supports over compensate that lack of power. **
