We have a military pattern M1 .30 Carbine with UltiMAK mount and an old Bushnell Holosight. Within the first few days of shooting the brand new carbine, the rear iron sight body came loose in its dovetail slot and fell off. The front sight has since shot loose. It may be that iron sights are an "old standby" upon which one can rely when the new, high-tech gizmos fail, but in this case it was the other way around. The Holosight has been transferred from one firearm to another, used in all weather conditions, subject to .308 rifle recoil, and it still works like new.
I own a new Winchester 1894 "Wrangler" .30-30. I had my kids out for a drive in the mountains, when we decided to take out the Winchester and do some plinking. We were missing terribly, only to discover that the rear sight elevation wedge had fallen out inside the case. It's a perfectly good rear sight design, but it got bumped in just the wrong way as it was being put in the case, and the wedge popped out, throwing the elevation off by a mile. Update; The Winchester's front sight has now come loose, and caused us to waste more ammo chasing a wandering zero. The rifle has fired a total of no more than about 200 rounds since it was made - not enough to equal one typical day of test shooting at UltiMAK.
Then there is the Mini-14 Ranch model we bought new for testing our M4-B prototype. The Ruger is a truly great work of engineering. The first day at the range left us crawling on the ground, looking for the rear sight after it had shot loose and fallen off. We had the same experience with a Mini-30. Ruger, to their credit, has since redesigned the rear irons.
I once had a brand new Beretta Tomcat pistol. It was a flawless performer, but it came from the factory with a drift adjustable rear iron sight that was so far to one side the bullets were hitting two feet from point of aim at 20 feet. I was able to use a brass punch to drift it into a reasonable position, but it was not something I could have done at the range.
A new SKS of mine needed a small correction in front sight elevation so the calibration marks on the rear sight would be meaningful. The split screw front sight post, which is a fine design, broke in half due to shock and recoil before I got the chance to move it. Bad heat treat apparently.
Of all the optic sights I've used, I disliked the cheap ones due to poor optical quality and quit using them, so none of them ever got around to failing. Left with the better ones, I've used them for thousands of rounds of testing, target shooting, plinking and hunting, and they have never failed, except one-- a relatively inexpensive telescope mounted on a 10/.22 sitting in the back of my pickup in a soft bag for a year. My kids had stepped on it and the cheap mounting rings were bent.
While optic sights can and do fail, it just happens that my personal experience with the reliability of optics has been far better than with iron sights. Please don't accuse me of claiming that optics are tougher than iron sights. I'm not. I have merely related my personal experiences. Others will not doubt have had different experiences. One thing is certain; Iron sights are not the end-all, fool proof, always-there-as-a-last-resort-no-matter-what aiming system that some might think. Understanding, caring for, and regularly practicing with your gear is the key.