Depends on your purpose and need. . . I am assuming context here is something for defense or SHTF. The rifles that I have for that purpose have backup irons with a red dot as "primary means". That said, I shoot with irons quite often and actually not nearly often as I would like to. I actually thought about an upper with just irons and dedicating some training time for only that rifle, including a carbine course. To me, the irons don't take up any space that could be used for something else. I have seen where some have conflict with a DBAL or MAWL, but I'm not fancy like that.
Regarding those going without BUIS, I've also seen that as well and seems to be more common. Frank Proctor is one that comes to mind, but I know others as well. The reliability is one reason that is commonly cited. However, also consider that there are a number of reasons that can render an optic useless, or less than useful.
I've seen even quality optics fail and fail spectacularly. A Trijicon Accupoint lose zero on anything other than 1x power. Multiple Aimpoints lose ability to adjust POI. EoTech (may have been knockoff) where the lens came loose. So on. Those were just in carbine courses where the consequence was mostly an inconvenience.
The only rifle that I have that I "could" use for defense that doesn't have irons is my upper with the LPVO. For now, I don't think I'll put a 45 degree irons or mini RDS, but because that upper is mostly for experimenting with the LPVO. At least for now.
AK and Galil sights are always on there.
You have a red dot on an AK. You heathen!

J/K, I have an AK with a RDS. Need to shoot that one more. . .
