I get the impression that the kijin (Shuna, Shion, Benimaru, Hakuoro) abilities aren't defined, logical, or consistent in any way.

They're just arbitrarily "powerful" that manifests in completely arbitrary ways. Fireballs, spirit bombs, super strength, super ninja, super samurai, whatever.

They're stronger than everyone else, weaker than Rimuru. Author forgot to bother with the anything more detailed than that.