I'm not sure Clark is really bad, per se. He brought Markstrom to stardom. I think it's more a matter of fit that ability. Not every goalie coach fits with every netminder. Demko is improving under Clark, but Holtby has remained the same as his last year in Washington--neither improved nor worsened.
The Miller example is a bit perplexing, as Miller was very good in Vancouver, especially during the end of his tenure.