The reason Emma can't affect Kick's mind changes between versions. In the original he's shielded. In the English version, it's because his mind is unstable from being completely nuts.
Telepaths cannot affect Androids, which are the U Men foot soldiers.
Mutant powers tend to appear during puberty, which makes them harder to control. Hormones making emotions run wild and thus making it paramount to having training to make sure they don't run rampant.