I have to imagine the level of discomfort depends on the car. I've never ridden in a vehicle with adaptive cruise control, but if it responds quickly to someone jumping in line, that could go beyond annoying to motion sickness if it keeps happening.

Having owned a car with this feature, it's very uncomfortable when it suddenly and unexpectedly brakes hard to allow a car in.

And it can be very confusing to the vehicle behind you.

Ah. I see. My 2015 Model S doesn't do that, it just gently increases the spacing until it is as it was.