Fairphones have awful updates for the firmware and drivers with months of delays. They end up with an end-of-life Linux kernel and hardware components. Fairphone 5 and earlier have end-of-life kernels which aren't receiving the vast majority of important privacy and security patches.

They're missing crucial industry standard hardware security features. They don't provide proper encryption protecting user data for the vast majority of users not setting a very strong passphrase.

The main hardware and update requirements for GrapheneOS are listed here:

https://grapheneos.org/faq#future-devices

We left another reply at https://news.ycombinator.com/item?id=49354807 with more details on how Fairphone doesn't meet those requirements.