Per permaculture sources I have read, conifers are supposed to rot too quickly to see long-term benefits. OK, so your hugelkultur won't last 20 years. No problem. Your soil will still be better than it was before the burial of debris, regardless of species, as long as it is not walnut or redwood or some other species that contains toxins and/or inhibitors.
As for mine, I threw everything, including our Christmas tree, into the hole. Most of the limbs and leaves were oak, so it's a mix. Everything rots together just fine in the forest. And plants seem to grow well over the materials, whether conifer or deciduous. The soil in woodlands is more fungal than bacterial. This is better for tree growth than vegetables (most farmland is primarily bacterially dominated). Since I did not bury a giant specimen and mixed it up, I'll plant without concern that the soil is unbalanced. I'll throw some compost tea onto the bed later on; and it will all eventually adjust itself.
While the arborvitae is not as ideal as say a downed maple for this application, it has to be better than nothing and certainly more so than a bag of chemicals.