is it flawless, no, but it does work in conditions I didn't find fun driving in. An example was two lane highway, at night, in the rain, with enough jokers with badly aimed lights or not dimming their lights. It did very well and let me feel confident looking away from the road if the oncoming car's lights were too bright.
I did not buy the FSD suite, I don't have a desire for that level of autonomy and I am not confident anyone will pull it off soon. I mean I have seen examples where they map it all but that isn't the same.
Here is a video from Cadillac showing your statement is false: https://www.youtube.com/watch?v=_rxW68ADldI&t=0m39s
Tesla will speed up or slow down and change lanes automatically. This is not the same. Also, Supercruise is only enabled in very limited circumstances. You can use autopilot anywhere even if it isn't officially recommended.
You are still required to engage the turn signal; Tesla will speed up / slow down to execute the merge, eventually, and will steer the merge for you but does not make the decision to do so autonomously (though it'll recommend a new lane to you with a blue lane line on your dash).