It is quite true that a number of heroes have base powers that deal strictly 2 damage and nothing else. In comparison to those particular heroes base powers, I would agree that his is more powerful. But on the broader spectrum of things, of the wide gamut of abilities present in Sentinels, the overall effectiveness of his power is not, I would argue, significantly more powerful.

My numbers might be off by a few here or there, but I just did a quick check on all of the currently available targets in the video game version of Sentinels and came out with ~291 targets overall (villain, team villain, and environment). Of those, ~179 of them are targets that inflict damage in and of themselves. However, I would say at least 25 of them are targets that either don't have enough hit points to live beyond 2 points of damage (The Matriarch's fowl), are immune to melee damage (Voss' ships), or only do damage in vary situational circumstances (The Dreamer's 'Tooth Fairy', etc.).

So with just a little over half of the targets inflicting damage that could be affected by Rebuke, I can understand how it might seem overly powerful. However, there are similar effects used throughout the game that are depicted on villain and environment cards ('Ashclaw Revlos' springs to mind), and I don't think that target is much more powerful than, say, Gloomweaver's 'Cursed Acolyte'. Similarly, it's not an unheard of, or even terribly difficult, feat to return cards like 'Throat Jab' or 'Lightspeed Barrage' continually to the tops of their owner's decks and continually use them round after round. I would argue that's a much more powerful effect (albeit at a somewhat equally more difficult to achieve set-up).

Overall, I would agee that yes, it is a slightly above average power given that the average is simply 2 damage. However, I wanted to design my character to be a bit above average. Nobody really wants to play an uninteresting or underpowered charactet, after all.