perlfather

I believe that the Audi ACC sensors are based on a single (older models) or double (newer models) radar units (24 or 77 GHz range) mounted in the front grill rather than on an optical device.

superswiss

This behavior is briefly touched on in the owner's manual on page 112 under the heading "Driver intervention request". ACC has a maximum deceleration rate. It's designed to follow cars that naturally accelerate and slow down. If the car in front of you decelerates at a higher rate, ACC will yield to Pre-Sense to request driver input or engage automatic braking. I think this is deliberately done to get the driver's attention if something out of the ordinary is going on ahead of you.

Brimstone

It can, if you're going 70 and they're stopped, I lose that game of chicken everytime and hit my brakes. With reasonable (~20 mph) differences in speed there's no worry, I've had it work fine for faster difference but its jarring, and there's probably a limit, which can also be impacted by curves/angles/hills.