Juice's Lecture Notes wrote:
Name (Games +/- .500, Career League-Adjusted FIP-, Career fWAR)
Tom Bradley (-6, 88, 17.5)
Fritz Peterson (+2, 93, 32.7)
Jerry Koosman (+13, 90, 62.6) <---The 68th-best career bWAR is only 13 games above .500 across 500+ career starts)
Chris Short (+3, 96, 30.9)
Bill Hands (+1, 90, 31.9)
Ed Halicki (-11, 93, 15.6)
Camilo Pascual (+4, 87, 52)
Bob Friend (-33, 89, 61.1)
Len Barker (-2, 85, 24.7)
Rudy May (-4, 94, 36.3)
Rick Wise (+7, 95, 44.9)
Bob Rush (-25, 86, 43.7)
Pascual Perez (-1, 92, 19.6)
That's a good list. Why don't we eliminate all the guys with winning records? As I've said repeatedly, there is no really good pitcher with a losing career record. There are guys who weren't bad. I've cited Joel Horlen myself. And I'd say he was better than any of the losers on the list above.
So let's go through them:
Ed Halicki was a swingman who was only a real rotation starter for a year or two.
Bob Friend was a horseshit pitcher and he played on some really good Pirates teams too.
Len Barker was a guy with overwhelming stuff. That's why he was able to strike so many guys out. But it's not just his W/L record that shows him to be less than good. His ERA was high too. He has a 93 ERA+ for his career. That just isn't good.
Rudy May might be the best guy on your list. He's a contender for the Joel Horlen award. And significantly, only a few games under .500 in a long career. He had a big year in the National League near the end of his career.
Pascual Perez was the very definition of mediocrity.
Bob Rush might be the poster boy for your argument. He never pitched on a good team in his entire career until he came to the White Sox when he was finished. Still, we can find plenty of guys who were good pitchers and pitched for bad teams throughout their careers without being a whopping 25 games under .500.