They are obviously not Buddha-word in the same way the Pali suttas can claim to be. As Richard Gombrich states: "I have the greatest difficulty in accepting that the main edifice [of the Pali suttas] is not the work of one genius." The Mahayana sutras in some occasion exhibit deep insight; in other, many others, they present a marked sectarianism, and there is a very obvious position of opposition that characterize much of the Mahayana's development of its signature doctrines.
Paul Williams' BUDDHIST THOUGHT, chapter 3, give an excellent discussion of the origins of the Mahayana and its texts.
Another book of interest is Text as Father: Paternal Seductions in Early Mahayana Buddhist Literature, by Alan Cole
. Here is what someone (WML) who has said on e-sangha:
The author analyzes the Mahayana sutras as works of literature constructed with very purposeful polemical agendas.
The threats of hell in the sutras are a direct consequence of the fact that the religion can only lay claims to authority on the basis of its texts. In other words, since Mahayana has no historical lineage to the Buddha himself, and since the bodhisattva ideal is unverifiable in the present life, the text itself must be made into an idol--an object of blind faith and worship.
The psychological manipulations engineered within these sutras are too numerous to describe here, but the author of the book performs what seems like a pretty comprehensive analysis.
The intense Buddhist fundamentalism in these sutras ("believe this text or you'll go to hell") is so utterly contrary to the spirit of the Kalama sutta; but this makes perfect sense because any tradition lacking historical or pragmatic claims to veracity must have textual claims instead. A textual claim to truth can only be founded upon a fundamentalism of sorts.
One can observe some interesting similarities in technique between the Mahayana sutras and the Book of Mormon. (Both arose lacking claims grounded in historical reality or empirical pragmatism.) As such, both of these attempt to gain confidence from the reader through emotional manipulation. This manipulation especially operates through techniques that cause the reader to feel as though they possess a profound personal relationship or connection with the text itself (hence the book's name "Text as Father"). This sort of polemical technique is epitomized at the end of the Book of Mormon, which says (to paraphrase): "If anyone is unsure about the truth of this book, then let them pray to God and God will tell them the answer." (I have to admit this made me both laugh and cry a little bit inside when I first became aware of it...)
Really, if anyone is interested in understanding the early Mahayana tradition from a polemical and psychological point of view, "Text as Father" is strongly recommended.
The comments about the Kalama Sutta and Mormon seems to be the commentator's own, not from Cole's.