First, I should say that I have played Federation through Commander 7, and Klingon through LC 8. Here are a few of my thoughts on the balance:
1. Tier one seems to favor Klingons in space, while being evenly matched on ground.
2. Tier 2 seems to favor the Klingons in space during disorganized pugs, but heavily favors federation when there are two organized teams. Ground is evenly matched with neither side having a distinct advantage, given that everyone has access to the same gear and tactics.