Two rating services in the past month have completed reviews of jumbo mortgage-backed securities issued in 2010 and later. The reviews led to numerous rating confirmations, some upgrades and no downgrades, prompting questions about whether criteria for rating jumbo MBS are too stringent. “I’d guess the rating services internally said ‘never again’ for downgrades,” said one participant in the jumbo MBS market. Officials at Fitch Ratings and DBRS ...