HWH vs. boiler for radiant heating

I saw a few side comments about this issue, but would like to explore it in more detail.

http://www.radiantec.com/systems-sources/domestic-water-heaters.php

Makes a case for using a HWH as the sole source for DHW as well as for radiant heating.

FWIW, I would be considering an open system (unless there was a compelling reason not to -- just starting to learn about radiant), with an over-joist flooring. The 1200 sq. ft. house has wood floors, is somewhat insulated (brought "to code" via our utility's insulation subsidy program), and is located in the Pacific NW near Puget Sound (it rarely gets extremely cold here).

Our HWH is probably dying. It is no spring chicken, and I want to be prepared with a choice for a replacement HWH if the nipple breaks off when attempting to replace a dip tube that is broken.

Anyway, thoughts about HWH vs. boiler, especially considering that link above, are appreciated.

Few WH are designed for continuous use. Getting one big enough for both heating and WH use will not be inexpensive, either. If by open, you mean that your potable water goes through the heating loops, don't do that. The cost of bronze pumps and special fittings to keep heavy metals from entering your potential drinking water are both costly, and may not be allowed in your area; not counting the potential health issues. Constantly adding oxygenated water to a heating system means much more wear; best to isolate the potable from heating.
A modulating boiler and indirect will likely be more efficient and last longer.

Why do I have to always throw Legionella out there into the mix¿

Something tells me that the only reason why it is thought that radiant flooring (especially staple up) can be suitable for DHW is the temperatures are so closely related. For a slab 120 degree water would be out of the question.