I haven't tried it yet, but I assume it absorbs hardware compatibility from the LineageOS project, which I use daily. I want to switch too but I'm not looking forward to setting everything up all over again. Maybe I'll do it this weekend.
Speaking from my LineageOS experience, an important point will be understanding which parts of the operating system come from the AOSP (Android Open Source Project) and which parts are proprietary add-ons which come from the Google Apps and Services. For instance, you will lose voice recognition. You will lose swipe-gesture typing. You will lose the Google Maps applet (used commonly throughout many commercial applications). The camera app that ships with more modern Google phones is proprietary and the one from AOSP is a bit more basic. Google Now will be gone completely. You will need to find alternatives for the Google Play Store, YouTube, Gmail (app), Google Drive, Maps and whatever other Google-branded apps you rely on. This hurts a lot less if you already do your best to minimize your reliance on Google.
I've been using CyanogenMod and LineageOS on Google Nexus/Pixel phones for about 8 years now and the hardware compatibility has been fine (assuming you choose phones which are officially supported). The bigger concern is whether the apps you rely on will work. The F-Droid app store (exclusively open source) has a lot of great apps in it which don't rely on Google services. Things like Lyft and Uber might not work without Google services installed though. While these can be added on top of LineageOS with the third party GApps image (at the expense of your hard-won privacy), I'm not sure if this is possible on GrapheneOS. By using GrapheneOS you are basically committing yourself to open source apps, and the handful of commercial apps which are deliberately designed not to rely on proprietary Google components. Personally, I'm happy with AntennaPod, Fennec (Firefox), NewPipe (YouTube), and an assortment of other apps from F-Droid. The open source map apps are nowhere near as slick as Google Maps, but I'll pay the cost in convenience for my privacy. The OpenStreetMap database they're based on is pretty good itself for roads and hiking trails, but it lacks the street address accuracy Google has amassed from their panopticon cars, unpaid reCAPTCHA challenges, and privacy invasions.
I used to use CyanogenMod/LineageOS for years, but with gapps. I still use F-Droid for most apps
GrapheneOS seems to have a sandbox for apps that need Google Play services, so it seems to be possible to use the ROM without losing all access to those apps, which is what convinced me to check it out further.
I have alternatives for most of Google's stuff though. I just wanted to see if there are hiccups for other apps when using the Play services sandbox
Ahh. In that case, I'm in the same boat as you as far as knowledge and predictions go. There are a handful of users on here who use CalyxOS and GrapheneOS though.
