Yeah, the power sinker profile is more contact oriented. It's just not much of a swing and miss pitch, so for that to be a dominant offering as a reliever, you're basically at the whim of the babip gods and uppercut swing hitters. That's not to say that they can't be usefull and cant get swings and misses, because they can, just not reliably. They might go one outing where they get 5 whiffs on 10 pitches to 3 hitters total, then not get another whiff for the next 10 hitters and give up 4 hits, 1 of which is a HR.
Some would say that could happen to any pitcher... well, yeah I guess, but when you miss bats consistently, you're far less susceptible to batted ball variance.