Short term results can appear finicky. A bad outing for a reliever will look bad, stats wise, for a long time. Small samples act that way.
But a bad outing doesnt turn a good pitcher into a poor one. I'd want him to pitch tomorrow and the next days.
Good pitchers don't change into poor ones willy nilly. Nor vice versa. There's nothing "finicky" about them, no more so than a good hitter having poor results on a day.
And since good bullpens are by necessity deep bullpens--at least 6 or 7 deep, ideally 8 or 9--they are very difficult to build. There aren't a lot of good pitchers available.
Why is relief pitching the universally sought after asset to add every trade deadline? By everyone?