I think Tidus' combat skills are the most questionable out of all the main characters. He's the flagship player of the Blitzball league in his Zanarkand, but that doesn't mean he should be able to fight monsters, maesters and Sin. Funnily enough, he learns in about 5 seconds.

I suppose Vaan and Penelo have a bit to answer for as well. How they're able to keep up with the likes of a war veteran in Basch and an ex-Archadian Judge in Balthier is kind of glossed over in the plot.