I wouldn't trust it either. I still look for merges and specific situations and if there are a lot of cars around. However it does a pretty decent job on it's own. I've found it changes lanes too often and there's no way to turn it off easily. I just go into the setting and set it to require confirmation and then ignore it. It won't let me stay in the HOV lane without annoying me.
I don't know how the car is supposed to learn, but I let it do everything on its own when I'm by myself to hopefully help it learn. Otherwise, I perform all the dicey maneuvers manually when I have any passengers in the car.
I always check before it does anything. I've noticed that FSD has issues with left turns. It cuts into the lane to late (it passes the opening and passes over the white solid line). Have you noticed that at all?
Today I was in a left lane in FSD and told Tesla to move into right lane. Just as I signaled, a car pulled into right lane from a highway entrance. The screen flashed red, Tesla stayed in lane (as it should) and I waited to either overtake the other car or for it to overtake me, then change lanes. Instead Tesla suddenly braked, hard. I accelerated and took over. Is this a normal behavior in such situations?