I wouldn't be so quick to underestimate the cats. The first one snuck up on Saki quite successfully. Their speed and initial stealth takes a lot of karma demons (or regular misbehaving students) by surprise.

As for the second, Saki warned Shun that they had released more than one, so he was on guard for it.

I think all Cantus users are meant to be incredibly powerful. At the very beginning of the flashbacks, the telekinetics were insanely powerful. The greater their level of self-control, the weaker they seem. They can accomplish great things rashly, in a panic, or out of desperation. It is the fine control that they are constantly trained on at the school. That is where many of them struggle. It probably takes very little concentration to hurl trees and set them ablaze, or wring a body like a dishcloth, but as David mentioned, rebuilding a glass jar requires a great deal more control to get things just right.