The history of ML pitching demonstrates that one of the most important keys to success is finding the right place for your arms to succeed.
Failed starters that became excellent bullpen guys included Rollie Fingers, Goose Gossage in 1976, Gossage joined the starting rotation, going 9-17 with a 3.94 ERA and 1.36 WHIP over 29 starts. He would never start another major league game., Mariano Rivera (short time in starter role under Buck), Joe Nathan who started in SF early in his career, Eddie Watt started 13 games in 1966 and never started another, Jose Mesa bad starter, good reliever, Jason Isringhausen, … etc
You also had the guys who were elite starters and then became elite closers like Eck and Smoltz