The abilaty to blow up stars is not something they do on a day by day basis. It takes a whole lot of jjuggs to do that and more important it takes a lot of time to actualy achieve the desired effect. During this time the FLEET is exposed and by all means the shivans might not have the required whatever they need to blow up a star if you take a few of the juggs out.
It's still something abnormal when compared to anything in the game. The collective mass (or size for the pedantic who want to insist we cannot assume mass without reference) of the Sathanas fleet is incomparably small to their target. A much bigger diference than between the Lilith and any other ship in the game. The other ship of that class, the Colossus, doesn't have that ability either. Should we say that the Sathanas is not balanced too?
Also the Mara is weaker then its terran counterpart. The game says so.
So if the game said that the Lilith was more powerful than the Cain or any other cruiser it would be alright? Wait, what is that in the FS tech description?
FS1 Tech Room Description
While far more rare than its counterpart, the Cain-class cruiser, the Lilith is one of the most ferocious cruisers we have had the misfortune of combating. It has extremely strong weapon systems, as well as a very thick armor which our cannons can barely penetrate. One for one, the Lilith can easily destroy any of our cruisers. It is for this reason that they should be considered a primary target for all bomber squadrons in the GTA.
FS2 Tech Room Description
The SC Lilith class of Shivan cruisers is a far-deadlier foe than the Cain class. Nearly impenetrable armor plating enables it to withstand tremendous punishment, enabling its nine turrets to lash out at warships and combat spacecraft with impunity.
We can all agree that shivan subspace tech is miore advanced then terran/vasudan tech . However given the rate at which GTVA tech is advancing the shivans might just find themselfs in the not so distant future outpaced by the GTVA in this area. And not just this area. Fotr all we know the shivans have always battled lesser species that did not really stand a fighting chance against them. For all we know they might of never known defeat.
The Shivan could have battled two ants and a mushroom, it still doesn't change what is presented ingame.
Also shivans dont battle eachother for all we know and dont have the constant need to improve themselfs like the GTVA does. War bring about tech. advancements. And when not fighting the shivans the GTVA is adept at fighting enemies from within.
You assume something we don't know about the Shivans, which even if it were true, doesn't change anything about what we've seen ingame.