You should be able to see the effects, no? At least in cameras (the light won't be flashing).

Maybe - Unless the bypass circumvents that. NSA did exactly that on Samsung (Tizen) TVs a few years back.

One of their hacks, IIRC, was that they turned off the standby light so it would appear the TV was shut off.

Well the thing is they are absurdly fine electronics. It is trivial to reroute so that the switch applies to the LED and not the camera itself (and some other trickery based upon reading its state while keeping it covertly operational). Said lie could still be detected in other ways of course but it wouldn't be a simple and complete fix.

To be fair, if I wanted to build covert surveillance I could also trivially make the switch appear to turn off the camera to commercial camera software but not actually disconnect from my covert software that I provide to TPTB.

Microphones are much more the worry; you can't cover them up like a camera, and are omnidirectional.

MacBook airs have tiny holes near the microphone. If it’s plugged, it’s very difficult to hear what’s going on.

Other macs may be like this too.