A key issue is propane vs electric:
- Electric: It's tough to get enough juice to keep up with any significant flow, so at best you get lukewarm water.
- Propane: It's tough to match heating output to the water flow.
We had one on our boat and after the first few uses gave up for anything other than dish water where we were filling the sink in one go. Unless you run a steady strong stream of water, it fluctuates from ice cold to scalding.
I do wonder if a small (maybe 1/2 gal) pass thru tank downstream would solve a lot of the issue. Yeah, it wastes little water but the mixing in the tank would cause the temp change at the outlet to be at least more gradual.