I am not sure if I should interpret this as "sql-backend is a second class citizen that shouldn't be used in production environments (i.e. think of virtual directories) because of its experimental stage" or take it as an overstatement been made on purpose mostly to discourage new users from considering an sql based engine for their main ldap database backend.
As noted in the manual page, back-sql is entirely experimental. It has no dedicated maintainer, and has seen virtually no work in 2-3 years. You use it at your own extreme risk, and should not expect help if you hit issues with it.
--Quanah -- Quanah Gibson-Mount Platform Architect Zimbra, Inc. -------------------- Zimbra :: the leader in open source messaging and collaboration