In the Princess Star Domain shrouded in dense Subspace Shadow, Spiritual Energy isn't a particularly effective means.
Not to mention the suppressive effect of the Subspace Shadow on Spiritual Power, even in combat, Spiritual Energy isn't very effective.
In battling the Insect Race Fleet, the greatest use of Spiritual Energy is lost, as Gu Hang doesn't need to tear apart the Void Shields — they don't have any anyway.
Of course, Spiritual Energy can still do things in other areas. But whether it's simply using Spiritual Energy to form thunder and bombard ships, or using a Spiritual Ability Field to rip apart ships, the results aren't really ideal. With Gu Hang's abilities, he can indeed achieve something equivalent to the power of an X-level heavy artillery with Spiritual Energy, but it's not very cost-effective. Instead of expending so much effort, it's better to fire a few more shots.
