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
[1] https://play.google.com/store/apps/details?id=no.nrk.yr
[2] https://en.wikipedia.org/wiki/Norwegian_Meteorological_Insti...
I might be understanding this wrong, but note that GrapheneOS providing Google Play services as regular sandboxed apps should not have anything to do with whether an app requires play services.
So, if you want to use an app that does require them, you would need to install the sandboxed-play-services from GrapheneOS's Apps store then install the (Weather) app you need. GrapheneOS doesn't preinstall any Google (stuff by default), so there is an extra step.
App compatibility should also be better with sanboxed-play-services compared to microG since almost all apps/features that don't require meeting certification requirements (e.g. Google Pay and certain banking apps) or privileged integration (Android Auto) should work just like on another Android family OS. Something you might run into is the hardened memory allocator exposing memory corruption issues in apps whereyou might not see the same crashing elsewhere.
Compared to any other mobile OS, there is no compromise software wise on GOS.