Its not so much as which side (though Vasari own the late game with Phase Missiles) is more OP then others as which unit is OP.  Bombers and Long range frigates have a tendency to dominate the battlefield (and the Vasari give Phase Missiles to both types) as they tend to hard counter the others  hard counter (bombers hard counter Heavy cruisers which counter LRFs while LRFs hard counter the light frigates which counter the carriers carrying the bombers). Throw in some flak frigates which counter fighters that can counter bombers (hard counter) and LRFs (soft counter) and you have a big ball of death that is unstoppable expect by a larger version of said death ball.

Bombers kill Star Bases.

As said above, sins suffers from the same 'imbalance' as a number of RTS games: Ball of Death. With a hard counter system, it generally ends up that you only need two types of units, maybe three, that hard counter the hard counter to each other.

Static defences are alright, but not great. As said by Ryat, bombers are the counter to pretty much all static defences. Especially at Supercritical Mass. The Carriers sit at the edge of the gravity well where the defences can't hurt them, whilst 1000+ bombers devastate everything within the well.

In single player this isn't so bad. That AI are generally... stupid, so that is where the SP imbalance comes from, however multiplayer isn't so much imbalanced as it is Big Ball of Death rush.