Privacy

This is a very common complaint, and the main reason is that it's way more complicated than it sounds. The duress feature is designed to immediately erase any chance to recover the unencrypted data from the phone by destroying part of the data used to derive the decryption keys (the other part being your pin/password, which can't be destroyed for obvious reasons). Any attempt to boot into a fake OS would be obvious to anyone familiar with the OS, thus making it effectively security theater. Not to mention the waste of space maintaining a fake OS, which would require fully featured apps and settings, all of which would have to be designed to pass verified boot, which means you can't actually destroy the original OS the way they do.

There's currently no known way to extract data from a locked GrapheneOS device, especially once it's in BFU, unless you've heavily reduced the default security measures, so the duress pin/password is likely unnecessary unless the danger of someone accessing the data is greater than the punishment for destruction of the data.

Also, the "big flashy rebooty show" is less of a feature and more of an unintended happenstance caused by the device immediately losing critical data and being forced to restart. The articles make it sound flashier than it is. It looks more like the device is powering on but the screen keeps disconnecting for a second, before going black.