And Feizhou, who seems to be towing your line, has already admitted he has no idea about Cyrus and has never used it.
Well, cyrus does add extra things to do according to its documentation. /var/imap needs to be infallable for cyrus to be trouble-free like the courier or dovecot. cyrus-imap adds this as a possible source of trouble whereas dovecot and courier-imap only have to worry about the actual mailboxes (assuming maildir, courier-imap only supports maildir) which are basically a structure of directories and files.