Well, there was the case where the the NSA intercepted hardware shipments (I think it was Cisco HW) to install their backdoors.
So it's something between a slippery slope and futile attempt unless you have a multi layer security from different vendors or roll your own defenses.
NSA, OTOH, intercepted the switches which would isolate high security networks (red/black separation) and bleed sensitive information with these enhanced hardware.