Micro Art Studio have released two hardfoam hills.
Those are more like plateaus than hills..
They’d work for impassible terrain.
Seems to me that sloped sided hills always have an issue with models falling over (at least in the 28-32mm scales). This looks like an acceptable compromise, especially given that there is a nice amount of detail on the vertical sides.
I’ve only had that problem with the really bad GW plastic ones. Everything else has had a low enough slope or good enough gripping surface for my models to stay well enough on it.