Anderson, Roark and and Garcia were all terrible washed up pitchers by the time they pitched with the Jays. You can attempt to polish a turd but in the end up you are left with is a shiny turd.
Matz is another recent success story as he has turned around his career nicely under the tutelage of Walker.
You can't blame Walker for all of Yates, Merryweather and Phelps being hurt and missing most if not all of the season. Chatwood was absolutely dealing before the sticky stuff crackdown. Romano and Mayza have both been developed into great late inning pitchers under Walker. Most of the failed bullpen guys have been waiver wire retreads, not sure if the pitching coach is to blame when guys who failed under other pitching coaches and get passed around through waivers ended up failing again as Blue Jays.