I think Advent varies so much because of how even the same units preform differently in battle, depending on the ability and experience of the player.

Advent, more than the other factions, depends on units complimenting each other in battle, micromanagement and different abilities being used in tandem to achieve results. Malice and Cleansing Brilliance is the most popular of these, get it with an inferior fleet and you can nearly wipe it out, botch it and you get rolled. So in the right hands, extremely powerful, but with someone who doesn't know how to use it, it won't turn the battle. Other examples like the use of the Guardians, positioning of Illuminators and so on also can be factors.

TEC and Vasari are more straightforward from my experience. In many battles the most micromanagement that is needed is choosing what ships to focus fire on, there are only a few abilities that you really need to use with care.

The end result I think that Advent can be inferior due to their weaker economy and tougher early game in the hands of some, while the other factions seem much better. With others though they can be very strong (some would say overpowered with things like Repulsion).

Advent needs abilities and synergies to compete with the other two's superior production abilities, I'm sure everyone knows. It's just how much micromanagement and deploying the right units can tip the scale when it's not just dps and hit points of ships that can change the dynamics of a battle, and results in some things seeming underpowered to some, and overpowered to others.

Things would be much simpler if everyone had the exact same skill level, I guess.

level 4 minimum into combat and 1.5 times experience makes for some epic pwning, but where the advent really shine is giving the player information. Eye of the converted plus culture spamming equals no-risk intel 1-2 planets away (great for watching the wormhole deep in your territory without tying up a fleet), then theres the clairvoyance ability for the revelation battle cruiser, when fully upgraded, the ability lets you look at a new planet every 10 seconds.

then theres always the 1-2-3 punch of a Radiance, Rapture, and the progenitor: Radiance uses malace to make enemies attack him, Rapture uses Vengeance on the Radience to give 120% damage back to the giver, and the Progenitor uses shield restore to keep the Radience alive. i havn't been able to find if the Malice ability of the Progiator applies to Vengeance damage too, but if it does...