To be fair, Medea isn't all that impressive either, legend wise. Ditto for Medusa too. I mean, sure, everyone has heard of Medusa the Monster, but that's not the form she's summoned in Stay Night. I'd say Cursed Arm is pretty obscure to.
Cu could definitely be a higher grade though.
Ehhhhh, this might be a lack of familiarity speaking, but the only thing I know about Romulus is that he founded Rome. Sure, it is an impressive feat, but it isn't really on the level of a lot of other Servants.