The biggest problem with tankless water heaters is that DIY'ers and
many plumbers don't bother with the size calculations. Sizing a
tankless requires estimating usage and combining that with water
pressure. The higher the water pressure, the larger the BTU's required
to handle the same usage.

Also, remember that tankless water heaters are not energy efficient
across the entire range of use. Sure you save the energy of storing a
huge tank of water. But if you run a dishwasher, for example, that
uses only a few gallons per cycle and you have high water pressure and
high BTU's, you'll get very INEFFICIENT energy use for those few
gallons. The burner will have to light up like crazy and then shut
down fast after only a few gallons.

Tankless heaters are most efficient for bathing and that's where their
energy calculations are made.

If they're sized right, they're a great improvement over atmospheric
vented tank heaters.