1. They're the only ROM project that actually focuses on improving application level safety. This is a bigger deal than a lot of people realise.
2. They offer installation remote attestation - again, worth using if you can.
3. Lots of drama with Calyx and GrapheneOS which is very hard to familiarise with. This is because the discourse is often deleted (this is the policy of the Graphene OS chatrooms) and so it is difficult to verify claims without pointing to another instance of deleted comments/purported harassment. If you can help it, I recommend to just try ignore the whole thing until they start screenshotting the actual harassment.
4. A lot of people talk about Graphene having worse performance than a lot of other ROMs but this is actually counter to my own experience. Graphene is consistently the fastest ROM I have used.
5. You may see people kick up a shit about how Graphene uses sandboxed play store and how that's a bad thing somehow. If you are worried, keep in mind you can still use Aurora if you want your install to be anonymised (but frankly I am not sure what the extent of the changes that Aurora makes). Similarly F-Droid is available, but is super weird about how they sign apps.
6. There are a LOT of updates. This is a good thing but it can throw you off if you're coming from another ROM.