Yeah one of the coolest things about Metroid and particularly the Prime games is how dark they are compared to most Nintendo games; the opening of Prime 1 for instance aboard the frigate Orpheon where you find the dismembered corpses of the pirates and their subjects, complete with scan logs detailing their injuries, which we can see carried over to Prime 4 in this recent trailer, is a brilliant touch.
Much like the Xenoblade series, they bring some invaluable diversity to a most light hearted first party catalog.