Stability is a function of the steering geometry, not whether it's OSS or USS. I think at least part of what you're experiencing is part of the normal learning curve. Now, the other part, bents are just plain harder to balance at very low speeds (2-wheelers, at least,) and long wheelbases more so. Most people have problems with them starting at maybe 4 mph, give or take a bit. I've climbed 20+% hills as slowly as 2.4 mph and not fallen over, but it takes a lot of attention to technique to stay steady at that (non-)speed.