I wouldn't rule out using a separate search engine. Yes, you'll probably have to keep the index in sync with the db manually but I found that it also gives you much more flexibility.

I personally use Solr with MongoDB and it works pretty well. Keeping the index in sync is not that big of a deal for me (depends on the application of course) and in exchange I get a full featured FTS engine that is probably more mature and has more features than an integrated one would have.