The arrogant assumption that one's own system is naturally superior to the native's way is again an indisputable characteristic of U.S. history. Another theme is the sincerity of religious motivation, in spite of the contradicting evidence of economic ambitions. This conflict between the philanthropic ideals and the exploitative motivation still underlies U.S. foreign policy. It is a personal account like a memoir, but it is also a letter to a powerful reader, not the general public. It has no literary pretensions, but the circumstances demanded rhetorical skill.
Style is also hybrid.
While most of these authors were educated well above the average commoner of the period, most of them were not trained in letters. Thus their writing is mostly unpretentious and direct. Most of these texts record personal experiences in the New World and thus have a ring of realism and direct contact with the earth and people. The audience then was specifically the powerful leaders. One can compare U.S. military reports on Vietnam or propaganda films on WWII like Victory at Sea to get a sense of nationalistic justification.