Fairphone is as insecure as most non-flagship Android phones. Make of that what you will.
GrapheneOS takes security very seriously. Your average desktop PC or laptop won't come close to their requirements. That makes GrapheneOS an excellent OS for people who want the security of iOS without the many downsides of Apple. Their patches reduce usability but make the phone more secure than Google's own, official Android build.
However, if you've ever used a Windows (or Linux) laptop, you've already experienced the kind of insecurity that GrapheneOS tries to prevent. No hardware encryption accelerators outside of the CPU, rarely any patches that roll out within a weak of announcement, firmware protection being basically nonexistent, no A/B updates, almost certainly no verified boot (even with Secure Boot enabled), and usually no firmware USB lockdown.