if the eplus table won't accept an item, but [u]ses in JEI show it enchantable, is the compatibility issue in eplus or the other mod?

example:
RedstoneFluxArsenal-[1.10.2]-1.0
EnchantingPlus-1.10.2-4.1.0.115

redstone flux arsenal's armor shows lots of uses in jei but the pieces won't drop into the eplus table at all.

OTOH, I was quite suprised to see enchants for the exu2 boomerang and the boomerang itself goes into the eplus table just fine, so I'm leaning towards the 'other mod' theory.

Yes, other mods and enchantments from other mods are supported by E+. Note that some items may be enchantable but not at an enchanting table, and some enchantments can only be applied to certain items.