At best it's not very reliable, at worst it's completely batshit insane. You'd be hard-pressed to find a bigger fan of Robert Graves than me (he's in Poet's Corner, for Christ's sake!), but that's not for his scholarship. Graves' "scholarship" is generally dismissed as fantasy, and it is. After the war Graves became obsessed with the idea of the "White Goddess," some sort of pan-European, pre-Indo-European mother/moon deity that persisted through the Indo-European traditions and whose rites were the basis of all poetry. It's pretty loony (Graves associates the White Goddess, for example, with all women and argues that all poetry is hymnal praise of the female sex) and just about all scholars simply ignore it as a piece of very petty prose. His adaptations in The Greek Myths and the commentary therein are both based on his belief in the existence of this White Goddess, and he has some...bizarre stuff in his commentary. His distinction between "true myth" and stories is pretty ridiculous and more or less arbitrary, and it's full of false etymologies, questionable connections and generally cranky stuff. In addition, though Graves was a trained philologist, he had been out of touch with classical scholarship since at least the end of the war, and it very much shows--Graves seems to think in his commentary that there has been no serious compilation of reference works since Smith's Dictionary of Greek and Roman Biography and Mythology (1844!) and he's apparently totally unaware of the existence of the OCD and other reference works that already had editions published by the time of the publication of The Greek Myths. That's maybe not too surprising for Graves--he had lived pretty much totally cut off from the rest of the world for some time already--but it's not a good endorsement for his scholarship. Read it for its beautiful prose and the rather nice "adaptations" of the stories (many of which are pretty seriously twisted or basically invented by Graves) that are in our texts, not for Graves' scholarship