The problems associated with tankless heaters does not apply to the new modulating heaters if their modulation is controlled by the water flow and temperature.

I do design many radiant systems around the tankless water heaters.
Several of them offer very high quality products and make very simple, efficient, affordable heating systems that will heat both the house and a endless supply of domestic hot water.

     We have installed several different tankless water heater in radiant heating  and snowmelt systems. Including six different manufactures, from France, England, Germany and Japan. All of them had to be replaced with either combo's or boilers.
     The theory that heating water instantly is more efficient than using a standard storage tank heater is not true. Most storage tanks lose less than two degrees per hour and unless the tank is outdoors the losses are simply subtracted from your heating costs. The reason no tankless heaters are manufactured in the United States is that they can not be built to meet the efficiency requirements of the US government. Any water heater built in the US must be at least 76% efficient, which is miserably low. In a heating system the warm water return temperatures cause the efficiency of the tankless heater to drop below 40%. Tankless heaters also have a pollution rating many times higher than 94%+ combo water heaters. Another problem is that tankless heaters constantly draw heated air out of your house, even when off causing your heating system to run more. 
   Although we do not use tankless heaters for heating we do use them for snow melt systems. Snow melt systems typically return very cold water which some tankless heaters handle very well with good efficiency, they do need to be engineered carefully to maintain proper flow rates.

To overcome the low flow problems associated with many tankless water heaters, use two pumps, one to pump water through the heater and another to pump water past the heater or better yet use a storage tank.