We’ve been exploring a thankless solution, but the company that quited us said it isn’t a good idea in our area because the ground freezes in the winter. We don’t live in a super cold area, but it does snow a few times a year and it can get into the single digits of degrees Fahrenheit.
Why do you want tankless? If it’s not for space and “endless” hot water, then tankless may not be right for you.
Consider a heat pump tanked water heater if you’re looking for efficiency improvements.