I think the most important factor for your climate is going to be "how much and how fast is this material going to compact?" Those of us in fairly wet climates don't worry too much about the
water storage aspect of hugelkultur, it's the long term availability of nutrients that we are looking for. But as the biomass breaks down and the nutrients are used up, the hugel will shrink in size. The first year I put in hugels, they shrank in half, and so I piled on more
wood chips. Palm trunks aren't very dense to begin with, so I could imagine them shrinking down even more. However, if they have been decomposing for a length of time, you may not have as much shrinkage to deal with than if you used freshly fallen
trees.