With system 76 you get not just the processor, but also the other parts. You know System 76 didn't change network interfaces to something new with no linux drivers mid stream. Or Wifi, USB chipset, some BIOS, or one of the many other non-processor parts that manufacture change all the time. If there are drivers the substitution doesn't matter to anyone, but often there are no linux drivers for 3-6 months (best case - meaning that it isn't hard to write the driver, in the worst case linux will never get a driver).

Note that for the above substitutions there often is no part number change so you have no way of knowing if the amazon/costco model will work even if it works for someone else.