Did you actually use it as a phone?

Do you have experience daily-driving it? Any prominent negatives?

If the source is fully open (it is) than detecting and disabling backdoors is completely possible. Not to mention the fact that other OS projects face the same risks.

If Google cuts development of AOSP in favor of some closed-source alternative, the GrapheneOS team could simply continue development of AOSP on their own.

If you define the goal that way, then you actually need to clear a much higher bar of making your own hardware. Personally I'd much rather maintain a long term fork of AOSP than have to design, market, sell, and support a new device.

I remember about 10 or 15 years ago somebody pointed out that a big chunk of the GNOME devs used Apple laptops, even at public appearances, and it answered a lot of my questions about the state of the project.

(and I say this as a user of GNOME)

Many have unlockable bootloaders (though the number is rapidly declining with Samsung closing up). But not many have relockable bootloaders. This is one of the things that grapheneos have set as a minimum standard, hence the reliance on pixels. There's a few other specific things that the titan chip provides which they rely on but the relocking is the main thing.

"unlockable bootloader" is the requirement to flash anyone alternative on the phone, yes, but the GrapheneOS you mentioned will support _any_ device that is "flashable" and secure enough: https://grapheneos.org/faq#future-devices

Currently it's only Pixels from 8 up.

Other alternative firmware projects don't seem to be too concerned about security (eg they don't support relocking bootloader, don't support secure boot, don't release patches for months), so they're not really in the same ballpark ALTHOUGH I agree that they still might be better option than stock OS on the device abandoned by the vendor.