I see this stated a lot, but I think it's exaggerated. My car's adaptive cruise control follows at about 2.5 seconds when on the maximum setting, which is where I almost always keep it, including in heavy traffic. People do occasionally jump in, but then the car just backs off a bit and starts following the new car at 2.5 seconds. It's not a constant churn, and it doesn't cause any significant delay. And this is in the DC area, with dense traffic and ridiculous drivers.

When people say that it doesn't work, I really think they're just saying that they can't stand having other people get ahead by breaking the rules, and they'd rather drive more dangerously than let that happen.