We all know Aftershock is way too strong. It's becoming such a huge problem, and this has really come to a head with Lux support being so OP with it. It is giving massive survivability to the CC mages that take it (Lux, Sylas, Lissandra and sometimes Ryze) as they all have shields/heals that synergise with the bonus resist. I personally believe Aftershock may need to be completely reworked or replaced entirely. Dealing with this could be the best way to balance Sylas and Lux support.
It's also notable that it is, to a lesser extent, also too good on the tanks that take it, but with the exception of Nautilus, Alistar and Sejuani, every Aftershock tank could do with a buff - especially poor unloved Maokai (I miss him :'( ). Imo it's probably the most overtuned keystone out of all of them, besides perhaps Conqueror. Much like Conqueror though, I will concede that it allows a playstyle that is fun to play and watch, which I imagine is the balance team's intent.
EDIT: I have made a follow up post where I discuss everything we concluded, and some really great solutions! https://boards.euw.leagueoflegends.com/en/c/champions-gameplay-en/eLEbXTQM-aftershock-2-the-solution