Most of the complaints are about the cheap manually adjustable units. These have 2 knobs, 1 to adjust the gas/flame and the other to adjust the water flow. Any changes in water pressure or inlet temp will change the temperature of the water coming out of the unit.
What you want is one of the computer controlled, self modulating units. The computer senses inlet temp and flow rate and automatically adjusts the flame to give consistent outlet temperature.
You very much get what you pay for when you buy a tankless water heater!