Frazzle is quite easy to land on dark neutral stuff in comparison to most other enfeebles neutral to the target. The problem is when you run up against mobs with enormous innate darkness resist, which makes sticking it on any mob not much weaker than you go from easy to extremely difficult, or even impossible, without the ability to immunobreak to get past it.
Darkness is likely the element with the most mob types innately resistant to it, so this happens more often than one would like. Almost all undead and demons have sky high dark resist, quite a few arcana do as well, and then there are a number of other miscellanous mob types in other families that also have really high dark resistance, like Belladonnas and Gnats.