I have used Matech Back up rear sights for several years with absolutely acceptable performance.
Once zeroed they stay adjusted, the sight stalk doesn't lock upright so it won't break off if you drop the rifle with the sight in use, the stalk simply pops down onto the base on impact.Adjustments are precise.
1. The drawbacks, they aren't cheap unless you get a GI to sell you one or give you one from DX stock.
Commercial offerings are around $150
Yes, not all military equipment is stolen, some units remove the BUIS and trash them after deployment and order new ones. Since the Military considers the BUIS as an expendable item GIs can keep them in many cases without breaking any laws.
2. The aperture is fairly small and many people don't like a precision peep hole on a back up sight.
You can open the aperture with a drill but the sight will no longer regulate to the distance settings described on the sight.
No biggy, sight in the rifle and glue the adjusters down, use as a fixed battle sight.
3. If you are gorilla minded when you tighten screws you can break the base securing screw, I have seen more than one Matech that the tighter is better minded have broken, it does happen so go easy when you tighten the screw.