The Smart Mailboxes that I'd created under Yosemite disappeared, and Smart Mailboxes don't seem to be working anyway. All my Rules were reset - the Rule is still there, and the Conditions are unchanged, but the rule is marked as inactive, and the target mailbox is wrong. (The latter is an old bug, where the target folder is silently changed to the folder mentioned in the last rule that you edited. It's been around intermittently since 10.7.)
The number of messages in an IMAP folder isn't displayed until you select a message in the folder. (In Yosemite it was calculated and displayed immediately.)