Strength in my eyes is gears. And people have been pushing just over 400wtq on them. However shifting forks, syncros etc. can and have broke. But mostly that's the drivers fault and it's not from to much power. You gotta take it easy and drive really well to make it last and not blow no matter if it's a stock car or way above.
It does have hill assist. Take your time be easy on her and you should have no problems.
A lot of people have had trouble because they choosed to launch a awd performance car like the wrx. Like anything if you abuse it you can break it.
But one mod I highly recommend to help with shifting- Group-N transmission mount- then your shifter bushings. On my 09, second gear always felt a bit vague- especially when down shifting. The transmission mount fixed most of that- I can actually feel where 2nd is now.
I've been worrying about the same thing. Just picked up my new rex (first manual, first awd etc.). I'm taking it as gently as I can on the transmission and just enjoying the hell out of the turning. My advice is don't worry about acceleration but just have fun taking corners at 40+ mph