I think there's a genuine difference between a pub audience and a folk club audience, and not all performers who do a fine job in one environment would transfer easily to the other. As a real example, one bloke came along to our club from playing in a pub band. Once he'd learnt that he didn't need to yell at the top of his voice and hammer his guitar, he became a real asset to the club, but the first time was painful. I can readily imagine the other side of the coin, that some of the gentler club singers would have no chance of a hearing in a pub. Childe Ballads, anyone? Not in my locals. Not without amplification, anyway. I dare say Tanglefoot would manage, or John Kirkpatrick, or Greg Stephens, but not everyone's in that category.
To outrightly condemn anyone who couldn't hold an audience in a pub seems to be setting the bar too high.