I installed the Currie conversion kit on my mountain bike this summer. The Currie kit has a non-hub, chain-drive motor on the rear wheel. One of the big pros of this system for me was the great torque. I'm currently living west of Glacier National Park in an extremely large national forest; there are very few flat areas to ride here. I wasn't riding my bike much at all before I added the kit because most hills here are steep and my knees were killing me. The Currie motor is a trooper and I've been having a ball riding the unimproved, gravel forest roads, many of which have quite steep grades.
There are users who love hub motors who claim great torque, but then they're generally using the high-end, expensive and powerful hub motors that will run you close to $1000. I think many of the hub motors that come standard with most generic Chinese-manufactured bicycles may not have enough torque to perform well in mountainous areas since they tend to be lower-powered motors to comply with e-bike restrictions imposed by the EU.
As for noise on a chain-driven, non-hub motor, I think it's over-exaggerated. I only use my motor for power assist and rarely have the throttle depressed fully. The wildlife out in the forest may look up as I pass, but I don't make enough noise that they bolt. In fact, I still have problems with the wild turkey who tend to cross the roads in number.