If your use for the sight really requires a "life dependable " sight, then you're really saying that you need a sight that co-witnesses with a BUIS.
Glass breaks, batteries die, electronics fail - the only real variables are the mean time between failure for various sights and models, and their resistance to damage. But at the ed of the day WHEN your optic fails, you'll need a BUIS.
Get what ever you want, just be sure you leave the iron sights on it, and that you can use them with the optic attached.
I've been happy with the Burris FF III in a few different applications. It will mount on a rail and co-witness with the sights on a number of arms. And if it won't, you can usually get a mount or a riser that will make it happen. Again, I'm not willing to bet my life on an electronic sight.