I considered getting one of these when I converted my electric
water heater to gas. One reason I did not is that if the power
goes out, which it does occasionally where I live, a regular
gas tank water heater will still provide me with hot water. A
tankless will only keep working if it uses gas for the heating
and a battery for the controls. This did not exist at the time
I was looking although it may exist now.

Err... We've got one that we go "camping"[1] with that
runs off a propane pilot light, with a mechanical flow-sensor,
when water goes through it, at some minimum rate,
the main gas comes on. That's all there is to it.

Oh, there *IS* a cold/hot dial, but as near as I can tell,
all it does is restrict the flow so that the water that DOES
go through is hotter. Not really useful when you're mixing
at the point-of use, anyway.

--Goedjn

[1] Well, ok, it's more like setting up an over-funded
refuge town than normal camping, but still...