The models that the credit rating firms (e.g., Moody's, S&P, and Fitch) used to evaluate the risk of the various MBS debt securities and thereby assigned a credit rating (e.g. AAA, AA-BB, or unrated) were

right on target, but only in the aggregate.

super models, and while as a group they were not so good at evaluating credit risk, they made up for it with their good looks and impeccable fashion sense.

poorly made models.

unimportant, since they all turned out to be backed by the full faith and credit of the U.S. Treasury.