The biggest drawback IMO, is its very limited capability. You need water, then you need to carry that water all day long and hope it heats up, then you need to shower before that water cools back down. And it's still limited. Once that water runs out, you have to find more and start the process all over again. I can source right from a creek or river and have hot water in minutes.
If I didn't score the Zodi for so cheap, the Helton seems like the clear winner. But I'm not disappointed with my setup al all. Very few things beat a HOT shower when you're on the trail for an extended period of time.
I can see where this product would have a place in really hot climates with ample sunshine and few trees.