It's quite secure against casual attacks, but a proprietary mobile platform has inherent issues wrt. withstanding even mildly sophisticated attackers, including mercenary spyware services. You still have a huge attack surface from all sorts of proprietary firmware blobs and hardware IP blocks that are running directly on the SoC. It's not clear that it's really worth even trying to secure it as opposed to just treating it like an untrusted toy.

well, a concerted attack could easily subvert the baseband if you have a few million dollars and the correct letterhead or private contacts.

GrapheneOS really wants the software in the phone to not pwn the phone. This is good. Its a different, and much more difficult problem to secure the connection to the telco, and the larger internet, because the transport is attacker controlled.

Think of it this way: Say you use Qubes because security is valued very highly for you. Even if you run Qubes, if your router is controlled by your attacker, what kind of a security guarantee could you really get for yourself?