I have a very long house, 3000 SF ranch and going with a tanked system just didnt cut it, wasted water and fuel since it took up to 5 minutes to get hot water to the kitchen and back bath. I installed an electric on demand unit for the kitchen and it works great, only drawback is flow since it is only 3/8" piping, but does well for the sink and dishwasher. One other side effect is on/off cycle, you cant turn the water on and off while rinsing, since it heat soaks and you get bursts of cold water.
For the bathrooms, there are 2 full baths back to back at one end of the house and a full bath at the other end backing the laundry room. For those locations I used the Bosch tankless water heaters. They work great! Like with the tank type there is a small delay in getting the hot water to the tap, but once it hits it stays hot until you turn it off and there isnt the heat soak issue as with the electric one. You can take a hot shower all day, or fill a jacuzzi tub or waterbed nonstop. My fuel bills have been cut in half for hot water usage, which is a great thing since I am on Propane. One drawback is multiple tap usage, it is designed for one tap only, but there are larger units that supply multiple taps as well.
When properly installed, they are just as safe as a standard tank water heater, if not more so for the power vented type, which both of mine are. There are safety devices built in that will shut the unit down in case of a back draft or some other type of overheat to prevent fires and CO in the structure, but they MUST be properly installed for those devices to function properly. As far as scale build up, after three years and very bad water from our well which has lots of iron and minerals, and sometimes the water softner runs low on salt, so the water gets gunky, we have not had any issues with them. From what was explained by the company, the way the heater works, tends to self clean the lines with the expanding and contracting of the pipes when heating water.
Good luck, hope this helps.