Your primary downside to detach/attach-to-new scenarios is that the new server might not be completely backward compatible. This has been the case in the past with some mighty well-known database offerings; on the plus side, it's usually well-documented in their release notes.
If you usually give up a great deal of time in stepping down from these scenarios to data transfer scenarios (backup/restore,bulk copy, etc.), detach/attach is a good candidate. I usually prefer to use a backup/restore, which insures, first, that I HAVE a backup and allows for the DBMS to reformat my database as it is restored, which feels to me to be more likely to work than a database either working perfectly in its new home or reformatting itself (if such is required) on attachment to a database server hosting a new version of the DBMS.