Ryan over at Totalsurvivalist.com has a great discussion thread about AR optics and failures. This has been hashed out many times on many pages with both professionals and mall ninja's alike. Optics fail, regardless of manufacturer or use. I've seen every manufacturer and optic made fail in one way or another. I've personally seen simple temperature differential fog lenses and make a an AR into a nice club. Rain can also cause some issues with some fogging. Ryan has a ton more formal military training than I do, and that's really important. The adverse conditions he's seen are a testament to the suitability of the RDS and AR platforms.
As with all electronic/mechanical devices, the more complex, the more issues they have. I'm no expert, but with 15 years of helping at a gun shop that is near a military training facility that trains the tip of the spear, you do pick things up from those guys. There is also a military testing/research facility nearby as well. I have the opportunity of interacting with some very interesting people. They have some interesting input on optics and use. I'm assisting with advanced classes here and there, but I'm still a ways away from teaching the high speed stuff.
I suppose optics are the first thing to consider. I'm pretty big on the Aimpoint M3 for close to a decade. In that time, I've seen every other manufacturer fail on my rifle or another. The only other Optic I've never had a failure out of is a Trijicon Tri-Power. I know and have seen others fail, by not the one I have. It's been run on full auto, 9mm carbines, 308 rifles and many other riles. It's not the most robust unit, but it works in most conditions. The M3 only has one major drawback, and that's the battery it uses is a non typical one, and non rechargeable. It's fairly heavy as well.
I know a plethora of military rifleman that swear by the Eotech's, and I know many that despise them. The only optic that I know of that everyone is fond of is the ACOG. It's ultra clear optics and mid range magnification is head and shoulders above most. I know you can get similar results with a magnifier, but it's not nearly the same. The single best optic I've ever used has by far been the Elcan Specter DR. Sooner or later someone is going to come up with a similar design for less money, and then I will go for it. Until then, I just can't justify the expense.
Another often overlooked issue would be BUIS. Back Up Iron Sights are critical IMO. If you have a Red Dot Sight failure, you will be guessing POI unless you have another method of aiming. All my primary weapons have BUIS with Trijicon front sight posts. In very low light conditions, this is critical to be able to aim with confidence when your target is in the shadows and doesn't allow you to silhouette your front sight post. Another thing all my battle rifles have is a quick disconnect for the RDS. If you break the glass or get dust/mud/blood on the glass your sights are useless as well unless they are offset. Being able to quick release your glass can be important. I've been in that position and it sucks. Raining like hell, sloshing through mud puddles and go prone into a bog and fill your optics with muck. If you use plastic backup sights, you better stick with airsoft.
AR platform failure points. My goodness how we can count the ways. It takes so little to make a fully functional rifle into a paperweight. I've found that spending the money on a mid to high end rifle generally makes good sense. The major manufacturers spend the right money on the parts that must work properly for reliable function even in abusive environments. I also test my rifles to failure with poor care just to see how well they can handle the grime. My main 5.56 rifle can handle between 2000 and 2400 rounds without cleaning before failure. I spot cleaning every 500 rounds or so does make it last indefinitely. Pull the BGC, shake it off and wipe it down with CLP, brush out the lugs and chamber with a brush, lube liberally and toss it back together. If I run it with a can, the less rounds to failure.
My backup gun will only run 1600-1800 rounds til failure. I've since changed out the buffer spring and extractor spring, so that should help it along. The Chamber in that rifle is a bit rougher than my main rifle, so I put some of the issue as being caused by the chamber slowing extraction slightly. Need to test it again with the changes to see.
The most common "fatal" issue I see in classes and at the range is popped primers. They fall into the works and generally jam up the trigger. This is a crappy jam because it's fatal and cannot be fixed in a firefight. I use reloads most of the time, so it's something I have seen often with my rifle. They can also jam up in the lugs.
The most common non fatal issue is a stuck case. A new extractor spring or extractor will generally fix this. The next common is out of battery failure. This is mainly due to a dirty chamber, wrong buffer/spring combo or bad ammo. Short stroking, not ejecting the old or not stripping a new one is usually caused by a dry rifle. AR's like to be wet with lube. A plugged gas tube can also be to blame, but I've never seen one happen with brass/non russian ammo. Bad gas rings can cause this as well. If you can stand your BCG on the bolt end with it extended and it will not drop to the unlock position without pushing down, your rings are good. You can fix it in the field by changing a single ring if you are in a pinch. That's usually enough to get it back running again. Double feeds are due to magazine issues or operator error.
Last year when I took a vehicle class, I had SEVERAL issues. I know better than to change stuff right before a class, but I did it anyway. My Geisselle trigger kept spitting pins, My castle nut came loose and I kept jamming cases due to my inability to properly clean the case lube off reloads I was using. I also only had a couple months on a 2 point sling I brought. I used to use the backplate for my rear sling attachment, but it would cause me issues when doing clearance drills. Had to move it back to the stock. I tested my RDS battery before I left, but on the morning of the class, it was dead. Luckily I keep several spares in the gun. Had it not been a class, and had been a real need for the rifle, without BUIS I would have been in trouble.
Mags cause more issues than all the other items listed here combined. I've seen all mags of every type and manufacturer fail. My best advice is to beat up and cycle thru the same mags you keep for duty/carrry. If you like Pmags, keep 10 newish tested mags for your primary kit and beat up a bunch of the same type and manufacturer mags in training. This ideology is two fold. It allows you to test longevity on the mags you plan to bet your life on, and it allows you to train with the identical mags you will carry. When mags go bad, it just sucks. You can fix GI mags, but plastic mags tend to split the feed lips and have to be trashed. These mags pictured below were tested and found lacking. the feed lips are bent and they are unusable in this condition. The second pic is a comparison after adjustment. Filing down the front edge also makes them more reliable and less prone to jam with abuse. I don't personally do so, but I know guys forced to use GI mags that do it with good results.
I keep a spares kit in my Armor and another in my ammo bag. Included in the kit are gas rings, a firing pin, an extractor and extractor spring, a small tube of oil, half a tooth brush and a stuck case extractor. That's about 25-30 bucks worth of insurance. I keep Red dot sight batteries in the Hogue grip.
The receiver you use is the heart of your gun. I've seen some really weird things with messed up lowers. Buffer tubes threaded out of line, pins out of spec, mag wells too tight/loose, lug cutouts too tight/loose, etc. I've had people bring me junk receivers from brands I've never heard of that will never work right. I've seen big name high production companies with issues as well, but I've never seen an LMT lower that was out of spec enough to cause issues. Their QC is excellent. I generally buy LMT casting and internals for my battle rifles because it's a known quantity. My Bushmaster lowers were all made by LMT many moons ago. There are many other great manufacturers out there with quality products. DPMS has gotten a bad name in past years, but they do make a decent budget rifle/parts. I don't bet my life on them, but my son's rifles are about half DPMS with Bushy L prefix lowers and Spikes lowers. I do have to true up the DPMS uppers to make sure the barrel is straight, but other than that, they do fine.
The buffer tube is the next critical item. I've sold rifle to friends and had them change out the stock and tube to never get the rifle working again. The guy blamed me, and asked me to fix it. He put on some cheapo chinese tube. The threads were cut off center so the buffer had tons of parasitic drag and it was rough inside. I fixed it for him, but learned my lesson. Never trust your life on chinese junk. It's fine if you like fixing your gun at the range, but not good if you intend to fight with it.