Now none of us know all the new little tricks that the other races have up their sleeves, yet i think that there will be one glaring issue. TEC have easy to reach superweapons. Vasari have planet eating titans. All factions are able to turtle very well with titans + starbase + fleet, and all will be able to get an economy rolling. This means that when planets do get attacked they won't last very long against a large fleet.
In my humble opinion, advent need to be able to increase their planet HP/damage resistance a full upgrade or two to have a chance. This will be needed unless culture has received a HUGE improvement (which I hope it has).
Also I hope the vasari planet eater takes some time to chew while its gobbling up planets. The bigger the planet, the longer it should take imo, Terrans/deserts should take a while. Its a whole freaking planet! And once eaten it is gone, forever changing the battlefield. As I see it the vasari fleet should be a slow "unstoppable" force.
Now for the advent planetary mind control... IMO you should be able to over time take over anything that you have fleet supply for. Yes even titans, but over a very long time (30 minutes of fighting in the same gravity well). Small groups of ships should be immediately taken over via use of this ability(5-10 at a time). A capital ship should be subjugated after ~7 minutes of fighting in the same gravity well. This ability should have ~5-10 minute cooldown and cost a lot of antimatter to limit its abuse. A fully loyal planet should take ~10 minutes to mind control, and a fully upgraded Starbase should take 10 min as well. It should be cast on the target, then a player can see a timer counting down when that ship is selected, at the end of the timer the ship is lost to the enemy. Perhaps add the requirement that mind control be only possible if friendly culture is on the planet. In this way advent super weapons will be relevant and also this would add another level of balance. The possibility of a faction having multiple titans is in itself not OP. The conditions for avoiding the mind takeover would be warping to a new gravity well to drop the debuff. This would be incredibly balanced, as it is hard to lock down anything, even a titan, when it is running for its life. If it runs the enemy would lose antimatter following it so the debuff could not be reapplied over and over again. This is balanced. The marza was a superweapon that was balanced as well. Developers please have confidence to actually allow players this kind of creative freedom. This helps replayability because it gives advent players a less situational powerful choice. Choosing a planet to mind control is very limiting and not that that amazing of an idea by itself and planets aren't as important and easier to acquire then they were before rebellion.
When the mind control ability is leveled to level two, all mind control debuff durations are reduced by ~30%, so 7 minutes to take out star bases/fully loyal planets, 20 minutes to mind control a titan, and ~5 min to get a capital ship. This could also reduce the recharge time on the ability, whatever the balance may be.
This can be balanced quite easily, and adds a whole new level of tactics to a fight, and really gives someone a reason to build a revelation cruiser. This ability alone would make the loyalist faction worth playing. Also, make the advent loyalist titan the Weakest HP/Shield wise of any of the titans at level 1 to make it need to run a lot early on so that the enemy could accrue other advantages to counter. Some tweaking and this would be a very fun strategy.
In this way the advent actually feel like they can mind control, and it is balanced skill wise via time. So the advent have a nice fleet deterrent should anyone try to snipe them. You have a "window" of vulnerability that you have to inflict significant damage to the advent fleet, and if you tarry too long you will be up shit creek without a paddle. This feels right for the advent, as the fleet synergy requirements would be large. This feels unique. Please implement this in some form. Advent vs. Advent fights would be more interesting as well(one of the most boring fights pre-rebellion) because the power of choice, each player will have a different strategy about WHAT to mind control. What pressure to put on the enemy?