I think my driveway peaks at about 17%. I did scoop off the needles once, but there isn't a lot of traffic to keep it swept clean. It is ok dry, but slippery when wet. So it may only be the extremes where traction becomes a problem. I haven't had any traction problems with more ordinary hills, at least for driving power. Stopping power is the other concern, but I don't like to stop going downhill

I haven't built a kickstand for my bike yet. I had thought about adding balance bars to the trailer, which might be sufficient for the added vertical support for hill climbs as well as giving it kickstand capabilities, if it could all be given the necessary support + flex. Bungees?