Both of those comments are valid. Many of the heaters cannot modulate the level of heat...it is either on or off. If the thing turned on with too little flow, it would cause the water to boil, creating steam and a very unsafe situation. You could put in a point of use water heater (i.e., a small tank), but that sort of defeats the purpose. Use the search button in the blue bar for tankless and see various discussions. Some love them, many don't. They also require much more regular maintenance than most other types.
The output temperature will vary with the incoming temperature and the volume. think waving your hand over a flame. The slower you do it, the hotter it gets. Wave it fast enough, and you'll barely notice. The things aren't magic, they can only add so much heat. If you are using a small amount, they are often plumbed with a tempering valve to cool it off to a safe temperature, but otherwise, yes, the output will vary, sometimes significantly. It depends on where you live...if you dont' have really cold winters and your incoming water is always warm (say S FL or HI or SoCal) then they are more viable.