I would like to preface this with the fact that we don't actually have Nether Cap yet, since it grows in the third cavern layer, and we have only reached the first, with a few natural pathways to the second outside our current useful range.
So, the deal is, Nether Cap maintains its temperature no matter what. That temperature is quite low, the exact freezing point of water according to arcane tomes of wiki magic, and presumably can absorb/destroy a considerable amount of energy before giving out and burning, though it isn't indestructible or impossible to burn {Objects are destroyed by sustained immersion in magma, but can be used as pipes to move it for a short time}. This makes it a remarkable heat sink material, since rather than having to radiate all the heat to somewhere else, or becoming hot- and therefore less effective -even before the point of real failure, it just makes the energy go away, possibly using it to fuel some vestigial biomagical process to lessen the affront to the laws of thermodynamics.
On the other hand, to get energy out of it you either have to harvest the small heat-energy present in the object, which unlike the Tiffany Aching Trick doesn't have the advantage of simple mass of harvestable mass- air/stone/water -to bring the yields up to swiftly useful quantities. This method gives you a guaranteed, constant output of a little power, enough to charge a small battery over a few hours. Indeed useful, but not within short timescales, which is where hand cranked or shaker generators shine, Metro style.
The other method is differential energy, I believe it's called, where you somehow get energy from the difference in temperature between a hot thing and a cold thing. I don't know how or why it works, but it's an actual thing in real life, so I assume it does work. The problem here is that you need a hot thing to work with, which assuming you already have the tech for the first system, leaves the question of why bother with the more complex device.