When I drove a manual, on a steep hill, I would use the e-brake process. As for the Rav4 rolling back on a hill when the Mazda6 didn't, could be due to the fact the Rav4 is much heavier. My Mazda6 was a manual so I don't know how well the autobox held the car on a hill though.
I think Subaru's hill start assist is automatically activated with a ball inside the brake line or something that when you exceed a certain % incline, the ball will roll back and block the brake line holding the brake pressure for you until you gas it.
Don't know why the 4.3's had such a complicated system that requires you to press the brake harder to activate it.
However, I guess it's to not give you any surprises, cause on my Kia, the hill start activates automatically, but I would never know until I release the brake whether the hill gradient was high enough to activate the system, so maybe that's why the Rav4 Hill start assist is user activated.
2009 Rav4 V6 4X4 Sport