How often do you flush them? Some require service twice a year or they won't last very long.
I had 2 installed because our house is very long and the kitchen is on one end the bedrooms on the other. Even with my old gas WH it took forever to get hot water in the kitchen. This is much better.
We had a HUGE gas waterheater and it started to leak. If it had been in the attic I would never have known until it exploded and ruined walls and ceilings. The tankless is in the attic now and I used the former WH closet for other things.