DVDVDV Is a shut down pixel with good password safe from all physical adversary?
An adversary who can freeze the SOC with liquid nitrogen, grind the top off of the chip, use an electron microscope to read individual bits out of the secure element, and use a supercomputer, can probably brute-force even a medium-length PIN or non-long password.
There is no magic device which is secure against all adversaries, thus Pixel devices aren't secure against all adversaries.
Plus real devices, including Pixels, including the secure element, can and do have bugs.
All of that said, given current knowledge a powered-off Pixel with a current OS and a good password is top-tier. "Rubber hose decryption" may be more feasible than breaking in.