Hello all! First post for me, have been lurking for a couple of weeks. I find this conversation really interesting. My perception, as some others have said, is that both types are good for different reasons.
I think concrete skills and trainable mental/physical abilities work best as single stats. This make sure you can be both smart and strong (probably at the expense of something else, but you choose your own character’s weakness through gameplay, rather than it being dictated by the game’s stat pairs).
However I do like opposed variables for a character’s philosophies, or their alliances in a 2-sided conflict (e.g. pacifist vs warmonger, kind vs heartless, Orcs vs Humans). Opposed pairs are also good when a balanced view is equally interesting to an extreme one, and a high value does not always mean ‘good’.
Pairs can also be good to describe the balance of the world around your character - like the China vs US stat in Choice of Robots.
I think we just have to make sure that the opposing pairs logically conflict with each other, so that it makes sense that, when one value is raised, the other must always be lowered.