By not reliable I mean were much more likely to need to be shut down to avoid something really bad happening. The second reactor allowed them to easily return

By not reliable I mean were much more likely to need to be shut down to avoid something really bad happening. The second reactor allowed them to easily return to base (or even continue their mission) if they were forced to shut one of them down. An American sub with a reactor failure would be more or less stuck, and if it were under the icecap or something like that would be in big trouble.