There have been plenty of accounts of eotechs failing and, to a lesser degree, aimpoints failing. The bottom line is that these are both man made objects and can fail on you at inconvenient times.
The cost depends on which BUIS you choose. Weight is negligible and really adds nothing to a gun.
Assuming the carbine is meant as more than just a range toy, I see no reason to not have irons on a carbine with RDS, and a whole lot of reasons why you should.