I considered one until my plumber explained to me that the units he sells govern the water flow to regulate the output temp. If there is too much temp rise required, it will pinch the flow off noticeably.

His claim was that they are really designed to raise the temp by a certain amount...maybe 60 degrees? Our input water temp is 45-48 and he said that if we wanted water much over 105 that flow would be reduced, for some folks it is too much of a restriction.

I can't say if all mfgrs heaters work the same, but it would be something to clarify before writing the check.