WordPress search doesn’t search everything, and doesn’t give you enough control over what is searched and how. Relevanssi gives you full access and full control, with plenty of filters and ways to make Relevanssi work the way you want your search to work.
Relevanssi customer service was fantastic. When the project I was working on got cancelled shortly after purchasing Relevanssi Premium, they were quick to provide me with a full refund without any hassle at all. Thank you Mikko and the Relevanssi team.