Depending on what you mean by "heavily", the idea that "most languages tend to gravitate heavily towards a head-final order or a head-first order" might be wrong. (Related: Head directionality parameter and adpositions in Finnish) In any case, languages with both head-final and head-first constructions like English certainly don't seem to be rare.
In the WALS chapter "Relationship between the Order of Object and Verb and the Order of Adjective and Noun", Matthew S. Dryer writes that
The order of adjective and noun is often claimed to correlate with the order of object and verb. Dryer (1988a, 1992) argues against this, however, claiming that AdjN order occurs as often in VO languages as it does in OV languages. It has been known since Greenberg (1963) that many OV languages are NAdj. What Dryer (1988a, 1992) argues, however, is (i) that NAdj order is even more common among OV languages than had been realized, AdjN order being more common only in Eurasia; and (ii) that AdjN order is more common in VO languages than previously thought.
There seems to be more support for correlations between certain specific "head-final" or "head-first" constructions. For example, Dryer says in the chapter "Order of Object and Verb" that
OV languages tend to be postpositional (see Chapters 85 and 95), genitive before noun (see Chapter 86), adverb before verb, complementizer at end of clause, and standard-marker-adjective order in comparative clauses, while VO languages tend to exhibit the opposite orders. The patterns are sometimes more complex than this. For example, while VO languages almost exclusively place relative clauses after nouns, both orders of relative clause and noun are common among OV languages (see Chapter 96).