Now, I've always said that there's a thin line between c&w and r&b, and like you pointed out, different of instrumentation exposes the connection between the two.

But even more than that, I think what make both genre similar is the emphasis on narration, as artists from both genres rely(or relied) more on storytelling.

Listen to Percy Mayfield, Bill Withers or even Bobby Womack, I'll not be surprised if a country singer, ANY of them, recorded a whole album based on their songbook.

In all honesty though, I wish more R&B singers combined the two genres again. Ray Charles did it of course with much success, but that was about it. Bobby Womack, Syl Johnson, Tina Turner, and even Stevie Wonder tried(Peddle guitar in "Too Shy To Say") however, not as the same impact as Brother Ray.