The first thing that I think of is that SQL is designed for relational schemas, which would involve JOINs operations as well as indexes that optimize for WHERE,

The first thing that I think of is that SQL is designed for relational schemas, which would involve JOINs operations as well as indexes that optimize for WHERE, GROUP BY, etc. Overall, you have a reasonable expectation of structure over your data. On the other hand, FTS is working on less structured data a.k.a. variable length text. Therefore, an FTS system would need to include token filtering methods (which the article demonstrates) and optimize for the relevancy of search results. Another challenge for FTS is to create a ranking for your results. I would imagine that Lucene makes it easier to tune and administer your own definition of "relevancy" and "ranking", based on your application's use case.