Whoever forced it to use binary only blobs, like those employed by every wireless network card, can instruct it to exfiltrate users data to be sent anywhere, be it the manufacturers' or whichever entity they're controlled by. Closed chipsets not using blobs aren't immune either as malicious firmware can be implemented in iron only.
Sadly, we can't completely trust any system without total software and hardware openness, at least in all subsystems that deal with sensitive data.
Operating systems need hardware to run; hardware needs drivers; many drivers are closed; backdoors are easier to conceal in code that is closed and unavailable for auditing.
That is not something everyone could do, but that doesn't mean nobody could do that if given enough motivation and resources.
If you buy a system that requires closed source drivers then that's your own choice. Nowadays far and away most consumer hardware will work without any closed source drivers. Except of course the big N.
I mean if that's your threat vector you cannot trust anything nor anyone ever about anything at all.
The dude you just shook hand with might have had a glove and stole your finger prints. The post employee you just gave your check to mail might be a impostor. The guy you're about to meet from craigslist might steal your iPhone and cut your thumb to access your entire digital life.
Ah but no, you're an average Joe and nobody gives a flying fuck about any of these.
Even if the software is totally open-source: unless you also made the hardware: it doesn't matter, unless you compiled all of it (and at that point probably wrote the compiler yourself as well as the os you're using) and installed yourself: it doesn't matter, unless you actually went though every single line of code and checked for yourself that every thing is safe (which you cannot): it doesn't matter
My point has nothing to do with absurd conspiracies, but that technically, I repeat :technically, given enough motivation and resources which probably only governments have, there are ways to defeat encryption by planting backdoors into closed firmware so that data is being read before it even reaches the encryption layer. It's very hard to do, but it's 100% possible. You think the USB driver managing your keyboard doesn't have access to all text you write, including passwords? Well, have it talk using a covert channel with the blob running into your network card and all your text is magically sent elsewhere. Again: hard, still possible.
Nowadays NICs only have access to encrypted packets, so such a backdoor would have to be in the CPU. I'm not saying such a thing doesn't exist, but I think it's pretty clear its use would have to be very targeted for nobody to have noticed so far (unless there's a grand conspiracy between all computers to hide clandestine Ethernet traffic).
