For me I think the issue is trust. Its not much different from what I feel sitting in the passenger seat of a car when anyone else is driving. I don't trust their driving so if it gets slightly trafficed, I start freaking out because I don't trust the driver. I also don't trust the ACC on my Subaru in more dense conditions so when it get busy and I start approaching the rear of another vehicle, I go into a mode where I start paying close attention to the approach thinking "at what point do I cut it off, and hit the brake?"