It doesn't make them immune to status effects. Haste speeds a character up, slow or stop can cancel this effect, as well as sleep can delay. Berserk means you lose control of the character and I think attack raises a little. Protect means you are taking less damage from P. Attacks while Shell means you take less damage from magical attacks... even cure. There is no stopping your characters from getting status effects and you can't remove them unless one of your characters becomes un-berserked.