My e-bike experience: I live in a high-elevation mountainous area on the east side of the Sierra Nevada. The dirt road behind our neighborhood rises 2,300 feet in 5 miles, and that is where the good trails start. Maximum grade is 20%, but most is about 10%. I did that route about 3 times in 10 years on my normal mountain bike, and it was exhausting, but do-able, given a few hours. I am a former bicycle racer (haha, 40 years ago) , and regularly pound out 15 trail-less walking miles a day for my outdoor job.
I bought an e-bike, after researching to build one (more on that later). I have a Giant brand, hard-tail, with 27.5" wheels, a Yamaha 250 watt mid-drive motor, and a 500 watt-hour battery. Pretty standard stuff, acquired with 10 hours usage for $1800. It weights 52 pounds and looks like a monster compared to my other mt. bikes. One important thing, this bike has a "torque sensor", not a throttle, which means that the motor only applies force when I apply pedaling force. There are 5 motor assist levels, something like 50%, 100%, 150%, 250% and 350%, which is the multiplier force that the motor assists with. Many of the name brand motors are torque-sensor type.
This thing is KICK-AZZ !!! Once or twice a week I climb the hill behind the house, typically climbing 3,000 - 3,500 feet, and running about 30 miles. This takes about a half-charge of the battery, sometimes 60%. It seems I have never pedaled so much, because it is so fun I find myself trying to keep up with the motor. Did I mention how much fun this bike is? I also now commute to work, instead of driving. I live about 800 feet above the city, so the ride back up the hill is more enjoyable now. I rode once to the nearby city, 33 miles on paved road, and 1800 feet of uphill. This took 50% charge. As I ride more, I use the lower assist levels, typically level 2, or 3 for the uphills. I certainly use level 5 on the 20% grades - that is what it is for...
I bought a second bike for my son, and a third for my girlfriend, moving to the $2,200-3,000 range, each with the Bosch motor. We go on great and long rides, typically about 3 hours long.
I was going to build a bike, after researching endlessly on Endless Sphere, and it looked like I could get a much better value in terms of both battery and motors, easily getting a 750 watt motor, and on a bike I already had. I knew I needed a mid-drive motor, because of the hills (wheel-drive motors don't use your bike transmission ("gears"), so I was concerned about burning one of these out, a common concern). There is absolutely no need for a motor bigger than 250 watts. My speed on dirt is controlled by the bumpy factor of the trail, and more motor force would simply be excessive and have increased battery drawdown. Sure, I see some kids going more than 12 mph on rocky places, but I certainly don't need that, and the 250 watt motor gets me up a 20% grade for 2,000 feet just fine, and at a reasonable speed (though I do pedal hard myself here). Regarding brakes - dual disk brakes are the only thing to consider, period. You will find yourself at the top of a lot of tall mountains, and you have to get back down. I do see a lot of regular mountain bikers on the trail and often compare routes. Typically, we are covering twice the miles on our e-bikes that they are covering by their self-motive force.
It is hard to imagine riding a regular bike again. I do like how light my old bikes are, but that is all. Consider a quality bike rack if you get one of these, and they are charging $350 for the good ones these days.
Enjoy riding. I have tried to share my experience as accurately as I can to help others with decisions. These bikes are easy to rent for the day, so do that, and be sure to give the bike a good work-out if you do.