Yes, it can deal with 3D stuff and multiple floors and stairs, as long as you set it up correctly (WED paths and regions - the region system is for bigger levels or when you know enemies won't leave that area - if they are supposed to move between different regions then there cannot be obstacles, think open world for example where regions are placed around houses or villages or forests). The number of nodes is not limited by my code, instead by Gamestudio limits/ max_paths or what that stuff is called (you would probably never hit that limit).
It finds the correct path in linear time so it's fine to use paths with a big number of nodes, however not necessarily the shortest path (in quants). It calculates the shortest path based on the number of nodes instead. Just place some extra nodes on long edges to equalize their "weight" in the pathfinding algorithm (- as a result you can make enemies avoid certain dangerous paths by placing extra nodes there).

Whoever thinks that's not acceptable that the path might not be the shortest 100% of the time... then you're probably not a game dev to begin with, at least not someone who fill finish and release a game. (which I know you are and you do, @jumpman - I got that feedback before though)