That's not it, though. Just yesterday I was doing Aurum Vale several times and interrupted all Eyes of the Beholder without any issues on the Coincounter. The third run of the evening had a tank struggling to get away from the 100-Tonze Swings in time, so I stunned those for him. Then suddenly I failed interrupting the second Eye of the Beholder on the same fight, despite the fact that 100-Tonze Swings happen nowhere near as frequent that you'll run into diminishing returns. I reaaally don't remember getting any resist messages, the Stun effect just didn't seem to exist. :/ Though it may just be that my memory was faulty. Suppose I'll take screenies next time I go there of the combat log.

At least Spineshatter Dive ALWAYS stuns unless the enemy is outright immune to Stuns, but I don't want to rely on that for interrupts due to the long cooldown and animation.