Eirikr70 because graphene is open source it's harder to compromise.
But a group of users trying to mitigate much more aggressively against attack vectors presents as a possibly more valuable target for attack vectors and Pixels have some closed source firmware.
They would likely try to compromise something closed source.
Graphene mitigates against attack vectors. It can't stop them because it doesn't control all the code on the devices and even graphene could have a 0 day or undiscovered exploit. The intelligent graphene developers mitigate against many attacks and it would be very hard for a non-state actor to hack a Graphene phone configured well.