I ran into that recently, PoppaGator. I agreed to play in a pit band for a musical and when I saw the book I about died. It was Dixieland style and mostly new to me. Fortunately, it was simpler than it looked. As you mentioned, they were mostly familiar chords with variations and transitions tacked on. I find a lot of true chordorhea in piano arrangements for folk songs, though. I guess the arranger thinks it can't be that simple. Or maybe they're paid by the chord.
Fair winds,
SKP