Giles was actually 12/12 when the Astros traded him. In addition to the well-documented struggles with the organization, there were two problems with his pitching:
1) The Astros were almost certain to lose any tied game that he entered. He was a complete dumpster fire in those situations.
2) Even when he recorded the save, he often needed so many pitches he was unavailable the next day.
Giles in 2018:
Save situations: 27 games, 26 saves, 0.35 ERA, 0.662 WHIP, 93 batters faced (3.6 batters per inning)
Non-save situations: 28 games, 0-3, 9.12 ERA, 1.784(!) WHIP, 119 batters faced (4.8 batters per inning)
And that was
after he started pitching better in Toronto.