I have found that power wheelchairs have the ability to go over terrain that far exceeds the specifications for ADA compliance.
I can go up much steeper inclines than ADA specifies. There are a few things to keep in mind, though.
When transitioning from a flat section to an inclined section, the angle of the transition is significant. Some wheelchairs can handle more acute transitions than others.
My first wheelchair is front wheel drive and had anti tipping wheels installed in front. When I would try to make the transition from level ground to a ramp when the transition angle was more acute (a steep incline), the anti tipping wheels would contact the ramp and effectively lift my drive wheels off the ground. Thus, I would have no traction on the drive wheels and was stuck like an upside down turtle.
After reading a bit, I discovered that many folks with front anti tipping wheels removed them. I did so and it made an enormous difference. I can now transition to a pretty acute incline without any problem. In some cases, I have to raise my footrests or tilt the entire seat backwards so that the footrests would be clear of the ramp. This is the case either going uphill onto a ramp or downhill off of a ramp.
I also learned that carrying momentum into a ramp is a good thing, so long as the width and straightness of the ramp can accommodate the necessary speeds.
Another thing I learned is that when going down a steep ramp or slope, Gravity pulls me so that I slide forward in the chair. To help prevent this, I always wear my "seat belt", which is really just a body posturing device. I also will tilt the seat backwards so I am sitting level.
Another issue I initially had with going uphill is that the wheelchair would stall way too soon. My first wheelchair has high speed motors, which reduces the torque with normal settings. NuMotion was able to adjust the torque setting, which effectively increased the motor compensation, so that I could go up steep inclines. There is still a limit to what I can go up, but the increased torque setting makes a big difference.
Finally, I found that slowly going over a threshold (or any bump) at very low speeds just did not work. I found that if I hit the obstacle with some speed (say, 1 or 2 miles per hour), my momentum would carry me over the obstacle. Of course, that was pretty hazardous for the wall I would run into when coming into our house.
Increasing the torque setting on the wheelchair largely, but not completely, solved this problem. I found that approaching the bump at an angle so that only one wheel was climbing over it at a time really helped. I still do that on a regular basis.
I have found that experience has made the greatest difference. I have ridden my wheelchairs a lot and the experience I have gained doing that allows me to relatively comfortably handle situations that I would have found impossible in the first month of driving.
Steve