Screw holes are badly made. I had 2 screw holes totally unusable. I would need to drill and remove metal elements that are blocking screws from a normal screwing. So immediately, 1U is totally unusable.
If not that, then top 1U would be actually totally usable on the contrary to other comments. This top bar/rod doesn't need to be mounted.
The stability is excellent. After mounting just 1U unit, it has become stable already. Now when I have 3U filled, there's nothing to complain. It sits on my desk, and even that small part of it is beyond the desk, it's super stable.
I bought this rack for two devices: Focusrite Scarlett 18i20 (second from the top) and Behringer Model D on the bottom (but not at the very bottom because holes in the rack ears didn't let me put it lower.