I can't speak for the previous gen, but in my VA (2017) WRX, I find the hill assist intrusive. It seems to still grab for a split second after I try to get the car going from a stop, which is annoying. It's also pretty sensitive and I feel it activating on what seems to be pretty flat areas.
I also live in an area with a lot of hills and I haven't had any issues with the car rolling back with the hill assist turned off. In fact, out of all the cars I've owned, this one seems to roll back the least by far. Rolling back is only an issue for me on very steep hills, for which I use the e-brake to help me out with that. I only find myself having to do that once in a blue moon. I turned it off maybe 5K miles after I first got my car and it's been off ever since.