While I've not been in battle, I did have the gen 1 MBUS and I found them to be durable. I'm of the opinion that they would probably do just fine in "battle". Let me offer up this theory...
Mapul doesn't use kid toy plastics. They use (at the risk of sounding like a tool) highly engineered polymers that aren't brittle and don't break easily. That said, you'd more likely bend, scuff, mar, etc the sight before it broke. So, you'd really have to be trying to break the sights... like, dropping the rifle from a multistory building onto concrete... even then, it'd probably be fine. Now, with metal, it bends. Highly durable, yes, but more likely to bent than break and, since it isn't elastic like polymer, won't go back to it's original shape. So, in theory, polymer backup sights aren't bad and just because they aren't metal doesn't mean they aren't battle ready.
Now, all that said, I had the gen one MBUS and didn't like them because of the bulk. The gen 2's have less bulk, but they still have more material to them than similar metal versions.
I wonder if Magpul will ever make the MBUS front sight with the HK style round hood? Maybe they could get away with less material surrounding the sight post if it was a full hood? Who knows.