I've determined that the issue of the accuracy of stun with the Gungnir Shock Spikes Aftermath is an issue of Enhancing Skill/Magic Accuracy.
I did three tests. One as DRG/BLU with 0 Enhancing Skill. The proc rate of Stun's effect is <2% over several hundreds of mobs. Then I subbed RDM which gives me 149 skill. The proc rate of stun jumped to 6.5%. Then I subbed SCH with Light Arts and the stun rate of shock spikes aftermath jumped up to ~12% with 300+ Enhancing Skill.
I believe this may be an oversight by Square Enix when giving Shock Spikes to DRG's Relic Weapon Aftermath. DRG has no native Enhancing Skill and since the proc rate of Stun is based on Magic Accuracy(usually via Enhancing Magic), the proc rate is floored for DRGs unless we either equip and maintain a VAST amount of Magic Accuracy gear or sub SCH and maintain Light Arts for enhanced skill. Would it be possible to base the stun rate of Gungnir's Shock Spikes Aftermath on Polearm Skill or give it a natural base proc rate that doesn't depend on a skill DRG lacks?
or...you know, change the aftermath completely to something better