Similar to this question, but not the same thing....
When shouldn't you use a relational database?
If I have let's say 20 columns of data and it's not going to change in size, the only way to quickly search on all of them, is to index every column. This takes up a lot of space, and causes inserts and updates to take a long time.
But if the alternative is to use some kind of text-indexing-and-searching engine that does basically the same thing with a more proprietary format, why not use a relational database?
If your text search index has to be modified every time you add or update any of the 20 data items, how is this any different from updating the equivalent index in a database?