I wonder if Mini have changed the hill-assist, or if there is just a big potential variation between cars (which would be poor for quality management reasons)? Having just got my second Mini. I'm amazed how much less the hill-assist works in the new car compared to the old.
The new car needs quite a slope to trigger it to operate, whereas the previous car would work on the slightest gradient, and the new car lets go of the brakes almost instantly whereas the old one held on to them for the best part of a second.
I've had a few 'rolling backwards' moments with the new car when it doesn't behave like the old one did - which shows you how used to the old one's hill-assist I had become.