The problem comes from having to mentally monitor an additional complex system (the ACC) in exchange for the meager attention savings gleaned from not having to closely adjust distance to the next car. That's "adjust", not "monitor". You'll still need to monitor distance to the next car either way, ACC is no excuse to drift off. For example, my non-Tesla ACC will randomly brake hard with no car ahead.
I personally use my ACC as an extra set of eyes. I set it and still manually adjust my speed using the +spd and -spd buttons on my steering wheel. If the car ahead changes speed and I somehow fail to notice it, my car will give me one more chance by automatically slowing down a bit. But every time that happens I treat it as an "oops" like airplane pilots do when GPWS (ground proximity warning system) alerts fire. It's great that automation averted the problem, but it became a problem due to the pilot's error.
