Cornwall is a good writer: in certain aspects; a great writer. His strength, in my opinion, is plot, when is episodic and heavily turns on dialogue and 'cliff hangers', making it very readable. Purple passages are replaced by sensory descriptions as his characters' needs are basic. Thus the language is just sparse enough for the plot to engage. He's the John Grisham of the rifle brigade and has Grisham's downsides too. For the Peninsular War enthusiast, the backgrounds are realistic and, following a chapter or two of a Peninsula history book, they make very compatable reading. The language and descriptions of relationships can be repetitive: there's always a lot of 'flensing' and 'flaying' going on and if you believe that human import lies with the result of conscious thought, then the lack of introspection can make the books seem of little consequence. I might only suggest that you try reading Kinkaid's first-person narratives of his life as a rifleman to see the number of accurate parallels to real life.
As for O'Brian: his dialogues are simply embarrasingly wrong. Most of his charcters are practical men engaged in war, yet their dialogue makes full use of the passive voice, unnessesary linking structures and a completely cock-eyed formalistic structure so totally out of synch with men with jobs to do. Sentances are filed up with adjectives and adverbs at random - presumably to give the dialogue a 19th centuary feel. All it does for me is make me think that O'Brian is trying to copy the fomalistic prose you find in period letters rather than in real speech. Furthermore, what passes for insight into his characters for me simply looks like random focussing on irrelavent details with a worrying penchant for relationship analysis. I'd rather let someone else read his books and watch a telephone-flinger in the screenplay.
Patrick O'Brian once said of himself,
"Obviously, I have lived very much out of the world: I know little of present-day Dublin or London or Paris, even less of post-modernity, post-structuralism, hard rock or rap, and I cannot write with much conviction about the contemporary scene."
His lack of grounding in real life and the character Stephen Maturin frankly irritates
Moreover, can this be explaned rationally?