Mineral build-up is the tankless water heater issue that arises the most frequently. You need to watch out for mineral buildup inside your water heater whether you use a conventional or tankless model. Hard water is water with a high mineral content (as opposed to soft water), and the harder your water, the greater the risk of harmful mineral buildup.
Too many simultaneous hot water uses (such as multiple showers running) can overload your tankless water heater, depending on its capacity. If your water heater is overloaded, it might have trouble providing the necessary hot water or it might even shut off. If this occurs, you should reduce the number of simultaneous uses of hot water and you might want to think about resetting your unit.