I've seen all kinds of answers here, use it, junk it, only for lightweight, and more. This is what I have to say on the subject: use what you are comfortable using as long as it is safe. Safe has two definitions in a situation like this, what is actually safe and what you perceive as safe. You do not perceive this as safe, or you would not have posted the question. You are seeking the "opinions" of others because of this doubt. If for any reason you are not 100 percent confident in it for the use you intend, do not use it. There will always be that doubt in the back of your mind if you do use it.
I personally would not use that setup for anything heavier than a medium utility trailer, and only locally. Listening to that thing clack and clang and clatter at every braking, acceleration, and bump in the road would make me absolutely batshit crazy on a long drive. I would buy a drop hitch that would raise the ball to the appropriate height, a good ball, and a decent padlock. That way when the trailer is stored I just take the knee-knocker out, clamp it in the hitch, and lock it in...poor man's hitch lock, and and you don't have to look for the correct stinger when you go to use the trailer. I also put a weld bead under the nut on the ball making it pretty permanent, the ball can still be tightened if needed, but not removed. Never weld the ball to the stinger, the metal can crystalize and weaken the hitch.
Just my 2 bits, Dave