Its true that tankless heaters have a general temperature rise range. So if you they say 50 degree rise, then if incoming water is 70, the heater will heat the water to 120. They usually specify that on the product.
There are easy ways of dealing with that, such as a solar loop, before the heater. That is, simply a couple hundred feet of tubing that would be coiled before the heater, preferably in a warm area, like a basement. If it could be exposed to sun, all the better.
If you live in a fairly warm area, or don't have well water, you really shouldn't have to worry about this issue.
One of the reasons I went with a small tank is because it will heat the water to a set temp, regardless of what the incoming water temp is.