It's really a downside of the Google app ecosystem and not GrapheneOS per se, but apps requiring higher levels of integrity per Google attestation (Play Integrity/SafetyNet) generally won't work. Intentionally breaking apps on "untrusted" configurations is basically the point of that feature, and GrapheneOS does provide the relevant services, but would need to be specifically enabled by the app developer.

Firewall wouldn't be necessary with GrapheneOS. There's a network toggle which you can use to completely cut off internet access for an app. As for the downsides, I would say close to zero. It feels just like a stock OS, without any kind of bloatware and a lot more secure

GrapheneOS is not rooted, so you won't have those issues.

GrapheneOS also gives you a Network permission per-app; if you uncheck it, the app has no connectivity, period.

Highly recommended.

No NFC payments with Google wallet.

You can get unlucky with your bank app but someone maintains a wiki of compatible banking apps

Android auto works OK.

One big downside is being limited to Pixel phones, without good reason.