A manufacturer’s Android can have special privileges for their own apps, and almost will certainly have special privileges for Google’s apps.
Graphene by default wouldn’t give special privileges to any app, so that’s at least a plus.
It’s true that it would be locked down, but you at least have a couple more controls over how locked down compared to a manufacturer’s OS.
For reference, in the US, Comcast only gives up to a /60 for residential connections. It’s still fine for most use cases, but it does feel a bit like doing a bit of penny pinching when you’re wondering if you have enough /64’s for how your network is going to be set up.