Keeping an external search system in-sync with the primary DB is certainly a pain point. The biggest problem is that the records in the DB will likely be normalized, while the records in a search store will not be.

You can "poll" for new records for indexing by using a query, but handling deletes/updates is the real deal breaker. You will then have to use both the binlog (atleast for mysql) and querying, at which point it becomes quite complicated to reason about.