The "free as in libre" part would make it much more difficult to create an undetectable MITM. You might have a much greater diversity of hardware, so they'd have to find and maintain more exploits. The full software/firmware stack would be verifiable by cryptographic signature. The hardware itself could be checked by looking for chips that don't match other units from the same lot, either visually or behaviorally, and even x-raying the components if necessary.