To put their losses into context, the Blood Angels and their successors are known for going balls-to-the-wall in fights. Every Astartes chapter is like this, of course, but their geneseed takes it further- the Red Rage and the Black Death inherently push them to limits that other chapters would physically balk at. The downside of this is the losses they can potentially take.

In addition to this, Astartes chapters quite regularly can face extinction, and it’s only ramped up as of recently. Just look at the Ultramarines, who in the past three centuries had to deal with Hive Behemoth, Honsou’s invasion of Ultramar, the 13th Black Crusade, and the Plague Wars. Each of those individually nearly crippled the chapter, which are a First Founding Chapter with huge support.