Not the person you asked, but fellow Graphene user here. I don't whole ass into everything so I still grab a few things from the play store, but I use Aurora instead of play. The details on apps and ability to install older versions of apps is fantastic.

I honestly haven't run into a con yet. It's been equally or more stable than even AOSP for me so far. I've been on it for several months now.

Maybe one con might be that a non savvy user won't know what to do with everything. Even if they don't though, there is so much that's just default in support of privacy and security they will be miles better on Graphene than on another platform.

I did decide to find a few alternative apps to replace ones I had used previously. Finding an alternative keyboard to swiftkey was the biggest pain but worth it. I got Heliboard and installed a swiping library and once I got the dimensions of the board dialed in it has been an excellent replacement.