The TEC's defensive options are fantastic
To be honest, all the factions have some superb defensive options at their disposal.
Their firepower is unspectacular, though it will do the job
Cost for cost, TEC actually has the best overall firepower of any faction. In fact, there are only two weapon types that have an advantage over TEC: fully upgraded phase missiles, and fully upgraded plasma. On every other count, TEC either matches or surpasses the other factions for damage.
Ever since the Illuminator bug-fix, early-game rush forces have been dominated by TEC's brutal mix of low costs, high survivability, and brutal firepower. Vasari can keep up with the Skirantra and assailant, but a good TEC player will push them on the defensive. Advent... good luck buddy, you'll need it.
allowing the TEC to counter the Vasari, who's ships are very difficult nuts to crack.
TEC really has three big problems in the vs Vasari matchup. The first problem is the subverter, which will shut down a lot of your firepower. The Cielo's buff is meaningless if your units are disabled. The second problem you face is bomber spam, which can cause tremendous damage to your forces and knock out capital ships before you even get a chance to use their abilities. The third issue is phasic traps, which will shut down your entire strike craft force when you go on the offensive, greatly limiting your options. Having counter-measures in place for these tactics takes priority over unlocking the Cielo.
I'm not saying the Cielo is a bad unit, but I find it's not useful enough to justify a 5-lab cost, and it's not going to have nearly the impact of a guardian, subverter, or hoshiko.
you can keep the pirates off your back with bounty on other players
Most people play with pirates off, especially in Diplomacy.
the Incite Rebellion ability is useful as well, forcing the enemy to moderately harden all their planets, not just their border worlds.
Be very careful of researching this ability. The attacks it sends are quite small and easily repelled by a modicum of defense, but grant significant amounts of experience points. You may actually end up making your opponent stronger by leveling up their capital ships, and this tech does not have an off button. I've actually seen people lose because they research this. This is easily the most double-edged tech in the game, and personally I wouldn't touch it with a 10-foot pole.