With that number of replacements, I would start to wonder if there is some application or setting you are restoring from device to device that is contributing to the issues you keep having.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
DO the Pixel's work at all before going into the bootloader loop and bricking? Or do they do it out of the box? I, too, would suspect an app or setting as it happens repeatedly. especially if they work initially and this happens during set up or restoring/adding apps.
We have two Pixels, purchased in November of 2016, and both are working well - we have not had to return either one.