The fact is, that Takagi units work very well. There are two problems. One, tankless hot water heaters as boilers--just not as efficient as a real boiler because, as others have said, it is designed to take cold water from 50 to 120 degrees, not 120 degree water to 140. Boilers do that better. And Two, the takagi just can't do this job longer than five to seven years without busting. And then you need a new one. HOWEVER, my opinion is that they are ok, and here's why. Math.
Do the math: A new Takagi every five years costs about $700-800, whereas a new condensing boiler costs $3,000-$4,000 every ten to fifteen years. And during that 15 years, there is mucho maintenance and occasional repairs. With the takagi, it runs great till it breaks. So on that basis, it's a wash.
I've used Takagis for about 15 years and changed them out three times one one house and four times on the other. I am financially ahead of all the other people who bought boilers, which are expensive to maintain and service. The new condensing models of boilers are extremely energy efficient, but they are not long term machines, very unlike the old school wasteful cast iron boilers that would easily last 40-50 years.
I feel okay about switching out the Takagis because they pay for themselves, and I like gambling that every five years when it's time to replace, somebody would have invented something newer that would work better.
I have hopes for the Navien And Rinnai combo boilers which produce hot water and also function as boilers, but so far, the reviews aren't great.
So it's not an easy answer, but for the expenditure of money and eaze of a switch out, tankless units like Takagi are fine. You spend maybe 10-15% more on energy, but you save a bundle on repairs. It's just one woman's opinion. (A licensed plumber woman.)