I complain about wasting precious dry camping water trying to get the water adjusted so it doesn't burn your skin off or freeze you.
My on demand system has a adjustment knob in the kitchen (isn't that convenient for showers). Once the adjustment knob has been set, the final temperature of the water depends on the flow rate (faster=cooler, slower=hotter).
So doing all this adjustments just to get the water the right temp wastes a huge amount of water. Not a problem if you have full hook-ups. Big problem when dry camping.
I engineered a hot water recirculation system just to try and keep the temperature from fluctuating when turning shower water off to soap up, and then back on to rinse. The recirculation system marginally fixes the fluctuating temperature problem and provides hot water to the shower sooner.
When I had a hot water tank system, I didn't have these problems.
If it was cost effective I would have a tank water heater installed.
This is important to me because I dry camp most of the time and have a very limited supply of fresh water and grey water capacity.