It's not racial balance when the arrival of one piece of technology completely changes the gameplay. That's called bad balancing.
The two big contenders are Returning Armada and the Novalith. When either of these are fielded the gameplay changes drastically for the worse. Things like planet development, culture, research, and combined arms are completely thrown out the window.
Returning armada with multiple phase stabilizers will completely fill a completely researched popcap of ships in roughly five minutes, all free. After that it is almost impossible for your opponent to destroy ships faster than they are summoned for you. Try attacking a maxed out pirate planet with a popcap of dark armada, after the massive 5-10 minute battle you will still be at your popcap of ships.
The novalith can one shot any asteroid and two shot most planets. The development costs to redevelop a single planet is rediculous, especially in terms of crystal. It takes a good ten minutes simply for the population to come back up. Even if a player somehow lets his novaliths get destroyed one after another it is still almost impossible for them to not be cost effective. Consider that they can be built on any astroid in the back corners of the system and cannot be stopped. The Krostura Cannon and Deliverance Engine can be good, but are much lesser effects and must be stratgically followed up by an assault.
In small maps it is less common for the game to go on nearly as long, and thus these imbalances are less apparant. In larger maps there is significantly less ability to harass, and unlikely that any player will lose before researching and particular technology he wants. In larger games a Novalith can destroy planets faster than your fleet could even jump to them.
The races are otherwise relatively balanced around each other. TEC has the fastest and strongest economy, Advent can utilize much more micromanagement to gain the upper hand, and Vasari has powerful ships and high mobility.