I build systems from parts. (I mean, most miners do, right?) Motherboard, RAM, Graphics, Hard Drive, etc. If you can do that, most people who can, know if a system they are building is secure or not.
I do too. I still have no clue if there isn't anything lurking in the BIOS code or even hard drive firmware. Think about this: some of the "reputable" suppliers like Newegg or Amazon resell returned parts as new. So unless you have some sort of super-secure supply chain and absolute confidence in the manufacturer you can't really be sure.