ACC generally has a 3-4 second time interval that it permits between you and the car in front of you. I live in SoCal, so a lot of my driving is on very aggressive routes. The 4-second gap is mechanically safe but it's practically unusable because it creates a void large enough to invite other cars to lane change in front of me. So when that car merges in, the ACC detects a violation of the safe braking distance and decelerates to reestablish the gap. I call it the "cut me off" loop when we're on trips.
And before anyone suggests that I start tinkering around with the settings, I have adjusted it and the damned thing just resets itself constantly.
The beauty of ACC is it lets your disengage mentally. You can be aggro if you want to with it on, but I found it's just not emotionally worth it to get mad at being cut off anymore in a car with ACC. ACC just handles going forwards and I'm not having to touch gas nor brake. If I'm not touching either, I don't have to panic react to getting cut-off, just make sure the ACC is handling it, and if that's all I need to check, vs slam on the brakes, then eh.
And before anyone suggests that I start tinkering around with the settings, I have adjusted it and the damned thing just resets itself constantly.