My experiences come from using Calyx for a couple years in the past and I’ve been using Graphene for a few years as well now.
You lose a couple of things. The main ones have already been mentioned by others (GPay and Android Auto). Theres some other stuff you might miss. Now Playing doesnt work, AI camera features, weather on lock screen, etc.
Most of the stuff you might miss are annoyances, and some can be worked around fully with other apps. Nothing too major. With microG or Sandboxed google play, none of the apps i want/need to use are broken. That wasnt the case even a few years ago on microG, but things have gotten a lot better.