Tankless is more susceptable to mineral scaling...think teakettle. Depending on your water, you may need to flush it with an acid (vinegar is often used) periodically to remove that buildup or it won't perform properly. WIth a tank, even as that scale builds up, in between uses, it can build back up to temperature. WIth a tankless, it all has to happen nearly instantly...anything that restricts heat transfer is a problem. Also, the colder the water, the lower the output temperature will be. Not much of a problem if you are in a warm climate, since it may be pretty constant season to season, but, for example, my incoming water temperature varies from almost freezing to 60 degrees or more in the summer. A tankless also has a minimum flow temp before it will turn on - making it tough to get a warm setting out of the faucet. They are more complicated, cost more to install, and, if you get a good quality tank, not that much more efficient, although they can be. Standby losses aren't what they imply unless your WH is in an unheated space.