Because shit happens.
Twice so far. Also haven't seen the battery life as advertised either but that may be more of an issue with the batteries. Both times Aimpoint fixed them with absolutely no hassle. I think it was a switch issue.
I have seen just about everything broken, bent, smashed, blown-up, and abused to the point of failure.
Everything breaks.
The most frequent issue I see with Aimpoints is the assumed battery life. They frequently die well before the advertised life. This can be due to all kinds of reasons: cheap batteries, batteries that were on the shelf for a long time, use in cold, use in high heat, fouling and corrosion, etc. The user can also impact run time by simply forgetting to dial the dot back down to 7 after daylight use. It happens.
Just as said before, glass is a vulnerability. Mud, blood, paint, water, fog, whatever can obscure your rear lens, or a hard impact can shatter the lens, so it's a good idea to have irons available for use; I prefer to be able to dump the optic and go straight into traditional sights, though offset irons are also a decent solution.
Never had one fail on me. With that said, trijicon is probably going to SLIGHTLY edge ahead in the reliability field due to the lack of electronics.
I have a T-1 and M4s and both have been flawless. However, I've witnessed another shooter's 2 year old T-1 go down during a recent carbine course. The windage adjustment just stopped working. The T-1 didn't look abused or anything like that. But as mentioned previously, shit happens. The shooter finished the first day with his BUIS and used the T-1 from my back-up gun on the second day.
I have a Comp M2 that will not hold zero any longer. I think I am going to replace it with a Burris M-Tac.
Do not have an m4, but both h1 abd t1 have held up great for me
I haven't had any problems with my Comp M2. The only Aimpoint I've ever seen fail was a T-1 during a carbine course. If I remember correctly the reticle wasn't moving properly when the windage knob was turned.
I have the utmost confidence in my Aimpoint and the quality is second to none. I do not, however, trust Duracell's QC nearly as much. Seeing as I depend on both products to an equal extent, I prefer to have irons.
I had a power switch problem with a Comp M4. Aimpoint Sweden fixed it fast but I had to pay a little as I was out of warranty.