EDIT Note: I wrote this at 1:30 AM while half-asleep. Take any absurdities I say hereafter with a grain of salt.
Honestly, if we go back to the days of yore, we have the three races, Advent, TEC, and Vasari, each with their own strengths and weaknesses as well as a focus in which they are supreme as well as a sub-component that added to them and made them less one-dimensional.
TEC: economic supremacy with the ability to spam cheap quick units. They get early trade and ultimately get Pervasive Economy and can use Novaliths for economic warfare. Various upgrades also contribute to their ability to build quickly, including things like Industrial Juggernaut or even Colonize+ on the Akkan.
Vasari: unparalleled mobility with reliance on research. It has always been the case that a late-game Vasari player is something to be feared simply due to the fact that Kosturas and Phase Missiles will mess you up. They can be anywhere at any time and attack however they see fit.
Advent: combat synergy and assimilation. The Advent at the beginning were incredibly powerful and between Illuminators, Guardians, the Holy Trinity (or the new Holy Trinity), and the Battleball in general, their fleets were incredibly difficult to take down while if they landed a single AoE on you, you would be doomed due to uncapped Malice. They also would win culture wars, though this was never much of an impact on the game because as far as gameplay is concerned, this is just a "build a building to counter them" mechanic and requires minimal action by a player to deal with. Clearly many of their tactics were OP or just bugged to be OP and so they received nerfs. Nerf after nerf hit them until eventually there wasn't a whole lot left. Illuminators were no longer undisputed kings of DPS, the Radiance fell to obscurity from the Malice cap, and various other changes occurred as well.
Barring an overhaul of culture which will simply not happen in Sins I (maybe Sins II, but idk how much more you could do with it before it becomes too 4X'y to be useful in a game with similar pacing), the only alternative buff for the Advent as a conceptualized race would be to buff their military. It is known that the Advent suck economically. Resource Focus is regarded under nearly every circumstance as a stupid idea and they do not have the economic boosts the other races have. The Vasari can scavenge resources from dead ships and the TEC can pull money out of nowhere. This is supposed to be the case I think. Perhaps we have been trying too hard to pound the square militaristic Advent peg into a round hole that we think would be balanced.
With the advent of Rebellion (no pun intended), races were split. TEC was split straight down aggression/defense. The Vasari were split down research/mobility. The Advent got a messier break. The AR are effectively a new race to themselves. They are no longer interested in assimilating or revenge. Their sole purpose is now to transcend existence as we know it. The AL was supposed to be the Advent we know and love, but really, because the AR got more military focus (Chastic Burst > most; Chastic Burst+Malice > all), the AL got relegated to the back seat: culture.
While culture may seem like a perfect idea for the focus of a race, it isn't in its current form. If you had to manage it more, it would be. It's a bit like diplomacy: build a unit; problem solved. The lack of depth and more importantly activity required by the player to use it as a tool against the enemy is the problem. If you spent 30% of the time you spent working on your fleet with culture once you got it up, it would definitely impact gameplay and would make the AL's focus on it work.
This of course would take exorbitant amounts of time that the devs don't have, especially this late into the product's life-cycle.
The result is that the AL too must become more powerful in combat to make up for it's shortcomings elsewhere. It's for this reason, that the more I think about the Unity Mass buff, the more I don't mind it. So what if the Advent have the best military? Wasn't that the original point? So long as they are sufficiently weak economically (so TEC can beat them), they should be relatively balanced (as Vasari have Phase Missiles which neuter Advent ships).
Maybe it's just me, but I think the formula that the devs had at the beginning: economy, mobility, military for the races was a winning one. It needed refinement of course, but it was something that could work wonderfully. I think that with time, this formula got diluted and the point of the Advent in particular was forgotten. I'm not saying they didn't need to be nerfed. There were definitely severe balance issues with Vanilla, but I think by now you know what I mean.
Yes, I'm biased. When I first got the game, I tried TEC and got my butt kicked consistently by the AI. I didn't want to try the Vasari because I felt they'd have some funky playstyle that I wouldn't understand, so I played Advent and adored the race. As time passed and nerfbats whacked them, I felt more and more like I wanted to play the Advent than I actually did if that makes sense. I wanted to be able to play them as much as I wanted, and this is especially true of the AL. Over time I shifted to the Vasari as I never could enjoy playing as the TEC very much. I'm sure this bias is why I've adjusted the Advent more in Rebalanced Races more than the other races.
Even accounting for the bias, I still think that this may well be a good direction to go in. Call me crazy, but honestly, the more and more I think about it, the more I'm fine with this change.
Anyways, it's 1:30 AM here and I'm rambling, so I'm having tons of typos due to sleepiness, so I'm going to stop now. I hope my convolutions made sense.