I made extensive tests with the Epiphany armor a couple of months ago:
- Karma determines the amount of mana you get when it triggers. At high level of Karma you get 20 Mana.
- Frequency is determined by the number of parts you use.
The Mana Burst triggers only when you receive direct damage though spells (curses etc don't work) and I have never succeeded with low damage ones. Even with a Frequency of 5 the burst is very rarely triggered: It can take 20-50 Flamestrikes or more to trigger, and after that there is a cooldown of (several?) minutes. If you use only 1 Epiphany part, you will likely take over 100 Flamestrikes before it triggers. You will have to pay attention to your Mana because there is no message saying anything.
Like many new mods, the devs seem to try to make sure they are not overpowered, and more often than not it results in something so underpowered that it ends up beeing completely useless. Epiphany armor clearly falls in that category.
I do actually wear an ephiphany necklace on my thrower, only because of the poor other choice for that slot on a garg dexxer (and the fact that the necklace doesn't loose imbuing weight from Mage Armor, you just loose the 20 free except resists compared to a crafted one). With just this 1 epiphany part, the property almost never triggers, and when it does chances are it is because there are 20 mobs Flamestriking you at the same time, so Mana is usually the least of your problems.
For all other parts than gargish necklace & earrings, the situation is even worse, because not only the property triggers way too rarely, not only do you loose the 20 except resists compared to a crafted part, but you also loose 1 slot and 140 imbuing weight from the Mage Armor property.
Clearly the best use of Epiphany parts is to trash them for clean up points
On a semi-related note, I'm looking for any precise info on the "Spell Consumption 1%" property of the Virtue Bane gauntlets/stonearms. I've been wearing them since months because I needed the 10 LMC on a char, but I have never noticed the property triggering, so I assume it doesn't give any message either, and is too underpowered to be noticeable even when it triggers, it it does.