The B-1B is so underrated because it has been such a disappointing aircraft from it's introduction into service.
It has one of the, if not the worst Fully Mission Capable rates in the Air Force. It was originally designed as a high altitude, high Mach dash speed strategic bomber, then was transformed into a low-level subsonic penetrating bomber.
It's first several decades of service were strictly in the Nuclear deterrent role, with no conventional weapons capability at all.
After the B-2 came online, it was upgraded to carry conventional bombs and use the Sniper targeting pod, and was useful in the W.O.T. as a loitering bomb truck, but still suffered from the lowest availability rates in the Air Force.
Maintenance per flying hour is a staggering 150 maintenance hours per flying hour.
The thing was a sexy aircraft, and was supposed to be a replacement for the B-52. It never fulfilled that role, and needs to be retired when the B-21 comes online.
Bias alert. My father worked majorly on B1 sensor systems.
He had a very different view of the B1, as a bomber used extensively but basically never mentioned in news reports, etc.