Current FAQ says the following regarding index-time field boosts:
Index time field boosts are worthless if you set them on every document.
see the FAQ.
I think, this should be changed to
Index time field boosts are worthless if you set them on every document and solely search on this field at query time.
Because, when searching on multiple fields, a match in a properly index-time boosted field will score higher than a match in a non-boosted field.
See this discussion on Hibernate Search forums.
Not sure if there are more places where similar statements are made.