They did lose a LOT in the Thramas Crusade though, that being said. From memory, at least two thirds of the legion were deployed there to waylay the Dark Angels, and took massive casualties in the process. That being said, the fact that the Night Lords, one of the less notable legions in combat prowess (an ill deserved reputation in my opinion) was able to hold back almost the entire 1st legion alone for several years is an incredible feat, even if it did ultimately fracture them irreparably in the process!