I did not realize that tankless were more of a leak risk. Makes me wonder if a "smart" water meter monitoring system is worth the time and expense. Since it seems to be only a matter of "when" and not "if" a leak will occur.
I don't know whether to blame the architect or builder, but many homes in my neighborhood had the hot water heater tank located in the attic (good during summer when solar heating provides an efficiency boost) but increases the risk of water damage. I got lucky that a smoke alarm was located a few feet across on the ceiling underneath and it went off at 4am one morning. Who knew that a smoke alarm could do double duty as a water leak detector and wake up the whole house? When the local plumbing company offered to put a tankless unit in the crawlspace rather than a big tank back into the attic it was a no brainer decision. So while I dislike not being able to get a low flow rate of hot water (without prefilling the pipe and wasting water in between) I do like that water distribution is kept under the floor.
When the old plumber explained to me the heat exchanger model in these units, it all made sense. To get the water up to temp that quickly without a tank reserve (on demand), the materials need to be very conductive and limited in mass to quickly pass the heat into the water. And those thinner materials have to endure more severe thermal cycling than a conventional tank does.
As he was installing our new cast iron Weil-McClain boiler several years back, he also told me why they love the new super high efficiency boilers over the conventional cast iron ones. It's because they make a fortune repairing them. They apparently are more prone to break, and the sulphuric acid they produce eats everything up. Our cast iron model is a 30 year boiler at half the price, only a few points less efficient, and will still be going when I am gone. The only thing it's ever needed was a new expansion tank this year.
I don't understand why some of these units don't have stainless or at least copper-lined tanks. But then again, I do ... cost.
Leak alarms and auto shut off switches are a great last line of defense with these systems. I'd rather deal with 50 gallons in a perimeter drain then a 3/4 horse well pump running all night into a finished basement.