I only have experience with regular AutoPilot but I want FSD to be a lot more aggressive than AP. What I find most nerve wracking is when it just sits there or seems like it can't make it's mind up what to do. Aggressive is preferable, then I can make the snap decision to disengage rather than wondering if I should disengage or let it continue to be indecisive.
When AP was young and new, one of it's most obvious faults in heavy traffic was the delayed response to the car in front taking off from a green light. Then they fixed it - even if the car in front had a heavy foot on the accelerator, AP would follow suit. I liked that. In recent weeks it has reverted back to it's old behavior, maybe even slower. It seems to take it's sweet time going from 0-55 mph and leaves a large gap even if the car in front is not particularly fast.
I'll find it the least nerve-wracking when they give me "Mad Max" driving style and the ability to de-tune aggressiveness if needed.