I have a BCM QD sling mount, but I have a BCM KMR-A rail and not the UTG rail. It could be possible that those reviewers have the screw set either too short or too long (and therefore drops out) when they were installing. The Keymod Sling Mount (KMSM) has a little piece of metal that goes onto the slot in front of the hole you are using for your sling. If it is too short that protrusion underneath will not make it over the rail and into the hole, and too long I already mentioned. See picture (from the BCM website) about that little piece of metal I was talking about.
That nub prevents travel rearward, while those screws (which are long enough to work, short enough to sit flush if torqued properly) prevent forward travel. I can not speak on the quality of the UTG rail, but these BCM sling mounts are good to go, I love mine: