@Mark_Harwood Thanks for detailed explanation. It confirms what I already understood.
As I see, it's actually not undone, right? E.g. if user-boost is provided for "first name", it evens out IDF-scores for every query-term and only then applies the boost for "first name" field (for every query-term). At the same time, user-boost is not provided for "last name", so it evens out IDF-scores for "last name" field and the boost "1" remains by default.
Also, I still don't get why can't we use cross_fields on big-free-text fields.