I think his focus is around RF based power, which I imagine is his reason for also not adding mekanism.
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Magneticraft power system is at all nothing like RF....
But, while I love mekanism, I am of the opinion that themed packs should not have overlaping mods. If he chose TE, adding Mekanism would just be another mod doing the same.
And on that point, I do not agree with having Magneticraft and Immersive Engineering. Both mods are very similar and fill the same role, early game tech(even if they are not so "early game", but give a sense of less techy since its huge multiblocks). Taking in account that magneticraft is in very early stages and a lot might change in the mod, and uses a diferent power system from all other mods, I am not sure if it has a place in the pack or will just cause problems balancing the "progress line".