Yeah there is no upside to blowing up Tarkus. It's a failstate without a game over.
Well, by doing so, the PC can seemingly unlock
Nova / The Gray Goo Armor without being forced to sex Anno - as Nova / The Gray Goo Armor is seemingly gated behind that specific Ghost deck quest requirement.
That can technically be construed as an upside to an extent (at least in my opinion), as being forced to do something that one may not necessarily want to do specifically for
gear upgrades is not necessarily fun design (again, in my opinion at least).
It is a pity though, that that's the only other way the PC can acquire
Nova / The Gray Goo Armor (as blowing up a planet is not exactly the most desirable thing to do), as they're a relatively strong set of gear that has multiple use cases - having them be gated is rather vexing (in my opinion).