Advent has a clear advantage in huge single-fleet battles, while Vasari has a clear advantage in small-scale skirmishes. TEC comes somewhere down the middle. This doesn't mean that Vasari can (or even should) avoid huge fleet battles or that Advent can avoid multiple fronts. It just means they excel under different situations. Always evaluate the situation based on your current economic and military position. If you've got a map where you'll have to spend five minutes to run from one side of your empire to another, keeping a single big fleet just isn't practical.
Advent is a tricky faction to play, but I wouldn't say they're "gimped". Their economic upgrades are weaker, but on the flip-side the savings a mothership will give you on your planetary upgrades are enough to mitigate those issues well into the mid-game. As I said in an earlier thread, Advent really aren't at a civic tech disadvantage until the other races start putting up four or five labs. By that point in the game, Advent can be putting up four or five military labs, and unlock some of their exceptionally dangerous units and techs like repulsion. Obviously Advent has to be far more aggressive into the mid-game, but they're well equipped for that.
