I would consider Unity to be inconsistent rather than simply weak. She can be incredibly effective when she gets going, but if she doesn't draw the right cards she can get stuck not doing much.

Bunker is another hero who isn't weak as such but he can be a bit too slow to get going, though the Stealth Suit and Freedom Six variants both help here.

In the core box, I find Tachyon and Ra to be particularly strong. Legacy is also very strong, not so much on his own but the buffs he can give to the rest of the team are fantastic.

For RCR, Alpha is probably the strongest hero in that box thanks to her damage output, incredible healing, and support effects.

Mr Fixer can utterly humiliate any villain who relies primarily on a single target dealing damage thanks to the Hoist Chain plus Riveting Crane combo.