Custom verified boot needs to be supported in hardware. But with most devices, you can use "fastboot boot" from an external device to start from an image that you trust.
Indeed, but in return it only supports pixel phones, sadly (considering they're not great value for money for custom rom purposes, and most of their added value is lost when running a custom rom)
Well, Google packages the pixel phones with their latest OS updates and pixel specific features like Gcam. By running a custom ROM you lose those. Its cameras mainly perform so well because of the big AI farms at Google.