Fair for Its Day: Buchan's writing can tend towards the jingoistic when read with modern eyes, but it's nowhere near as bad as some other authors writing at the time; in fact, when compared to works such as Bulldog Drummond, Hannay and Buchan are downright progressive. Greenmantle in particular shows detailed knowledge of and respect for Islam.
Foe Yay: Stumm in Greenmantle. Examples of it include Stumm's man-on-man fight in his rather frilly drawing room and intruding upon Hannay while he's getting dressed. (It's even worse when you read it.)
Nightmare Fuel: Mary's description of what she's going to do to the bad guy in The Three Hostages makes the good guys quake in their shoes.
Values Dissonance: Buchan's writing can tend towards the jingoistic when read with modern eyes, and there is use of language that would not pass muster in these more enlightened times. Hannay more than once uses the word "nigger" to describe the inhabitants of the African continent, and "white man" as a synonym for a person of honor and good moral character.