I have had people taxi onto the runway when I was on short finals twice and a helicopter cross in front once, all would be classified as sudden from my perspective! On the other hand I fly the downwind leg with 65% cruse power and prop in cruse mode if I can. I fine it off on X wind and can then abandon the approach at any time I am below circuit height.
I agree very strongly that down wind checks should be kept to the minimum and lookout should be a high priority.
Rod1