Scale will always occur except in extremely unusual circumstances that only cover 10% of the human population who obtains extremely low-calcium water in extremely wet regions like the Pacific Northwest of North America.
For everyone else, scale is a fact of life. Certain minerals dissolved in the water, for instance calcium carbonate, have solubilities that decrease with temperature (odd when most things get more soluble with temperature).
This means cold water that is flowing through any device that heats it will precipitate these bizarrely-behaving minerals onto the device's heating elements.
This is simply a fact of water physics. Harder water precipitates more material, of course, but even seemingly soft water will become overloaded with calcium after it reaches 200F in contact with the heater.
Removing the calcium before it enters the heater is challenging because there is a lot of water going through. Like thousands of gallons a month. This is why water softeners are enormous. But they work.
You need some way to visually monitor the scale. Certainly scale is problematic once it significantly reduces thermal conductivity and/or when it constricts flow rate. As for when exactly "too much scale is too much" is hard to define. Perhaps 1/4in thickness of scale should prompt doing something.
Scale can cause acoustic noises like popping, crackling, gurgling, due to poor water circulation and excessive temperatures due to poor thermal conductivity. Certainly if you hear unusual noises time to descale.
When descaling is required, use an acid-based descaler. However minimize contact between the acidic descaler and the heater because acid is corrosive to metal. Any acid solution put in your water heater should have nontoxic acid corrosion inhibitors in it. This is something I am currently researching: https://chemistry.stackexchange.com/questions/111546/simple-acid-and-corrosion-inhibitor-combination-for-descaling-steel-boilers
