The way I see it is that Kindred are collectors of the dead. They come to collect your soul at the moment of your death when ever that is FATED to happen. They don't actively go searching for people to kill and I don't think they chose when you die. Rather, they're their to ensure you DO in fact die. I like to think of them as death incarnate or personified. Are they powerful in the sense that most mortals cant "defeat" or avoid them? Sure. But they don't actively use their "power" other than for they're intended purpose. For example, say if Ascended Azir defeated Xerath, Kindred would come to collect Xerath's soul, but it doesn't mean that Kindred defeated (or even could defeat) Xerath.
For a few reasons, I'd say Kindred aren't the strongest beings in Runterra. For one, death has been defeated, even if only temporarily, by a few champions (Karthus, Azir etc.), so they're not invincible. FauxSchizle also stated that they themselves also have an "end". I'd personally say they're are quit a few champs more "powerful" than Kindred.
I would say as of now, Bard is without a doubt the most powerful being we know of in Runeterra.
