User blog comment:Xilinoc/Ch. 599 - Too Early To Win, Too Late To Know/@comment-24153155-20141002220434/@comment-11051098-20141008144919

The Sternritter doesn't play fair, we all know that, but I'm just saying it's not a good reason to say that they were easily taken out, when Kirinji couldn't even land a hit on them because of Nianzol, and Shutara was (almost) rapidly attacked by 2 elites whose powers we don't know.

Regarding Ichigo and Kenpachi, I assume they were SWP because of their potential. Ichigo has a true Zanpakutô now and his full power unlocked, and he effortlessly fought and held off 4 Sternritters at once. Kenpachi effortlessly killed 3 Sternritters in the first invasion, and now has access to his full power too as well as his full Shikai, strong enough to demolish a meteorite.

We don't know the full extent of their power yet. It might be a stretch, but they could potentially be close to Royal Guard level.