Also, either way, what kinds of plants can you grow without irrigation or rain for a full summer? I'm guessing only ones whose roots go deep enough into the pile?
Exactly. The apex of the hugel is fruit trees and perennials. with annuals filling in as they can. Zepp Holzer, the originator of the word, scatters seed mixes with great numbers of different species while at the same time planting out many fruit, nut, and other trees.
And what's interesting is that the roots of short and mid height vegetation join together, creating a dense mat of roots and fibers, that contain a surprising amount of mostuire. When you combine that with spongy soils, buried wood, and shade they provide localized cycles of fluids and humidity. Overall it makes for much more resilient and productive food systems. Though you have to pay more attention to what you are doing.
If you plant one by one, neatly spaced apart, you won't have this type of nutrient cycling and moisture uptake.