What's the destructive power of the Rasengan about?
Well, Rasengan being an incomplete jutsu, had no elemental chakra nature until amplified by Naruto Uzumaki.
As you've said, Chidori being of Raiton (Lightning Release) electrecutes the user upon hitting them. If hit with a Fire-Style jutsu, you'd burn. The damage received goes along with the elemental chakra nature the jutsu is of. But as I've already said, Rasengan is an incomplete jutsu and had no chakra nature, simply a spiral ball of chakra.
But because Rasengan is chakra compressed in a ball, spinning and/or rotating at incredibly high speeds, upon being hit with it, a wound that is sprial appears wherever the enemy was hit. This was shown when Naruto hit Kabuto with a Rasengan. On other cases, the enemy is sent flying.