So the MBUS sights were a financial based decision. I disagree with the "good to go" comment. MBUS sights are plastic and fold. The fixed BUIS is a far better option if you're concerned about reliability and a folding metal sight would be a close second. You also mention that you leave the rear up as a habit for "just in case" which makes some sense. Again, a fixed metal rear would have been a better option. If ones optic is destined to fail, a plastic sight will be the first to go.
On my Rimfire AR I don't mind running Rear MBUS.... For my front sight's ALL of my AR's have fixed A2 front sights. As I see it, the MBUS' have been as reliable as any other rear sight I've had INCLUDING a Troy Rear Folding BS which was about $200.
I'm not saying I don't like my Troy Rear Folding BS... Because it really is a great sight... But, when I consider that the MBUS is just as good, maybe better because deployment under stress is easier, and that the MBUS ran me < $40... It's hard to say the MBUS is unreliable.
As far as fixed sights being more reliable... I used to think so, which is why for a very long time I only ran fixed irons. After having run folders for a very long time as well... I believe that both types are just as reliable and even on my .223 AR's I would not be apposed to replacing any of my carry handles, A1.5's, or other fixed irons with folders (even an MBUS).
When I said that an optic will ultimately fail sooner or later, I'm talking from pure experience. Sooner or later your battery's are going to die, maybe your optic will die, etc... Something on it will fail.
I've yet to see any iron sights need battery's replaced, I've never seen the electronics in any set of iron sights fail, I've never seen the glass on a set of irons crack, I've actually never seen any sort of failure with ANY set of irons to date. When you consider that I've seen EVERY other part in an AR fail at one point or another, that's something. I still maintain that when the dust settles, your Irons are probably going to be the last thing to fail on your gun.
The first thing will either be your optic or something to do with your bolt.