Cards I feel are overpowered:
Valley of a Thousand Rivers - Generally, an Alliance/Mage/Legend deck is going to at have more than their opponent. Giving away a random facility for 1 gem? That seems like the fast track to abuse, especially considering the secondary victory method for those decks is resource victory.
Cards I feel are underpowered:
Spectral Dragon - This card requires 90 of the two most 'valuable' resources. It's secondary benefit(+20 gems) will be rarely useful, as most of the time, 150 damage will kill an opponent, so it's benefit is minimal.
Now look at Dragon Squadron: 70R. 20 more recruits, 40 less gems (a more 'valuable' resource) and 50 more damage.
Also, Spectral Dragon is undead. An expensive card like this will generally be drawn before you can play it. As undead, any common Holy card can discard it. Spectral Dragon will get little use, and even less actual play.