Over and over we have clients who are not satisfied with AC search results. Some have paid for a third party service like Nextopia or Hawksearch. I'm not sure of all the reasons, but we do have a block of sites which sell books and consistently have bad search results because of the way book titles are worded. We have been able to work around some of that by removing words from the stop words.
Here is one comment:

Their SQL statement is broken. Essentially the search is evaluating the entire string rather then each word.

For example "Tapestry of Faith" should look for all books with the words "Tapestry" and "Faith" regardless of placement or order. The search is instead looking for "Tapestry Faith" as a single string of text.

Another example is the book titled The Seven Necessary Sins for Women and Girls. A search for "Seven Sins" should return this book, but doesn't because it's looking literally for the string "Seven Sins" rather than the words "Seven" and "Sins".

If I remember prehistoric times correctly, AC 5 had a feature that allowed you to search for any of the words. I do remember that when clients upgraded from AC 5 to AC7, we started to get more complaints about search.