There are two appliances that use the name “tankless”, a boiler with a tankless coil and a direct-fired water heater. I will assume you are referring to a gas-fired unit that heats water in coils within the appliance, but is not a home heating boiler.
These appliances often have inputs around 100,000 btuh which is enough to provide 2.5 gpm of hot water. There is no storage of hot water so they can take care of a shower, or a washer, but not necessarily both at the same time. Their major failing is in trying to take care of small loads, such as brushing teeth, filling a cup of hot water. etc. the flow valve can not reduce input below about 1/2 gpm.
They will clog from hardness quickly. European and Japanese owners are used to having the device disassembled and cleaned yearly, Americans do not.
If gas is available, there will be considerable savings, enough to pay for and replace the unit every 3-4 years. If the device is electric, the bill will be as high as your electric heater bill. If gas is available, I recommend a gas water heater for savings, forgiving operation and less maintenance.