My last startup failed simply because my co-founder and I couldn't stop arguing. There's a fine line between compromising on an idea in order not to clash with the other partner to over arguing and defending your idea. We couldn't find this line and decided to stop. From talking with other friends in startups seams like that's a main reason why most of them failed, yet no one like to admit it or openly talk about it.

I wish there was a way to due-diligent co-founders like you can do with business plan.