Tankless water heaters last 20-30 years in comparison with the average life of a tank of 10 years. All the waterways on the tankless are copper, brass and a few hard plastic parts. There is no tank to fail. During its long lifetime it's like a few parts will need to be replaced along the way (draft inducer fan, electronic ignition part, a sensor, etc.), just like a modern furnace, which also has a very long life and need occasional repair. The tankless can be repaired when it breaks, in contrast to a leaking tank which needs to be thrown away in the landfill.
Tanknot Tankless has installed over 700 tanklesses over a broad geographic range. We are able to do this because we very very rarely need to return to service the units.