I agree with your assessment of their acting abilities, yet I like several of their movies.
Both Costner and Wayne always played basically the same guy. They were paid well to do that and when you needed that kind of a guy, they were perfect, but they had limited range as actors. I too have liked movies featuring both men. Costner in 'Field of Dreams' or Wayne in 'Liberty Valance' for example.
Kevin Costner in the 'Big Chill' was able to fully come to terms with the character played and demonstrate the depth that he could bring to a roll.