I rented one and was less than thrilled with it. There is a lot of trust put on those two rubber feet. Go out too far on the edge, it will slip off. Too far back in the step it will cause it to almost lay down. The other one out there have the roller blade wheels that drop off the edge locking the truck from going any farther forward. In essence, putting it at the optimal distance to move to the next step. I haven't tried that one.

I'm going to build a sled similar to the one in this thread. My stairwell is exterior so I plan on building a track with 1&1/2" PVC and sled runners made from 2" PVC split in half to ride on the round PVC rails. I'm going to use my utv and winch as the hoisting mechanism.

I think he's saying the stair nosing overhang is not perfectly parallel to the stair riser. In essence, one rubber lift foot could be in perfect position while the other one would be at the edge or not on the tread at all.