Lets talk about ADCs!
Prefacing this by saying it isn't an ADC-hate post. Just a logical assessment as to why I feel ADCs may be providing frustration for some players. Disregarding the new runes, this is focusing on what the recent seasons have provided for this class.
Supports have been put into a MUCH stronger place today than they were seasons past, meaning the role they primarily SUPPORT (Marksman) find themselves in a generally safer position when doing their job. As a result, that fact alongside ADC items having a lower cost over the course of time + the accumulation of gold being a bit easier (if I'm not mistaken) leads to a faster powerspike. EDIT: I've also heard many complaints regarding RFC+Stattik, am not personally sure of the power behind that combo though.
SOME of their items got changed from less components to more as well, making gaining power in short periods of time easier to do if they can afford to Recall. I also feel it necessary to mention there's also the standard First Tower Blood, incentivized in Bot Lane due to a less-sturdy-turret down there (not to mention 2 Walking Gold Bags + Dragon).
ALL OF THIS dials back to handing the ADC their power at a faster rate, and it doesn't help that most of their items (core or not) come with the entire set of stats they desire; meaning their diversity doesn't come at a big cost for base stats - only for sacrificing certain item passives. (Yes, I realize ADCs are more Gold than EXP reliant) For nearly all of S7 and (as expected) Preseason, there are many posts expressing frustration regarding ADCs feeling as though they spike earlier than what the player is accustomed to; unless there are factors I'm missing/am ignorant of, I feel these may be some of the reasons as to why people are experiencing this.