I can attest this is pretty accurate.
A lot of the Android apps you find in the F-Droid or Aurora store can be installed, but won't work since most require you to have Google Play services enabled which Graphene sandboxes by default:
https://grapheneos.org/usage#sandboxed-google-play
A perfect example is when I was looking for a weather app and most of the popular weather apps were a no-go simply because they required Google Play services. I ended using Weawow Weather instead of several other options like Weather Underground, Accuweather or WeatherBug.
EDIT: I'm currently using Graphene OS on a Pixel 4XL