sausage I would say it depends on the person being comfortable with using Android in general. If I personally had struggled with using Android UIs in general, I would have struggled with using GrapheneOS as well. GrapheneOS' UI is closely based on the Android Open Source Project. I personally find it easy and comfortable to use.
That being said, Android variants can have very different UIs. I personally find Samsung's UI more cumbersome to use than GrapheneOS'. To me Samsung adds more clutter to its menus that is slightly confusing to me.
One of my reasons for using GrapheneOS is that its security features very rarely cause me usability issues. Unless an app crashes because GrapheneOS catches a security issue in that app (such as memory corruption), I don't have to think about it. There are simple toggles to disable some of GrapheneOS' exploit protections for the problematic apps, if you choose to.
Google Play Services are not included by default, but can be easily installed by opening the app called Apps, tapping on Google Play Services and then Install. This will also install Play Store. Google apps can be downloaded from the Play Store if desired. The vast majority of them will function just as well as on Pixel's stock OS. The most notable exception is Google Pay's NFC functionality. It's a choice by Google to require a Google-certified OS for that functionality. The app "Google Personal Safety" also does not work. I can't think of any incompatible Google apps beyond that. I'm writing this using Gboard.