In general there are several ...

In general there are several pros and cons for using RO makeup in lower pressure boilers where traditionally softened water is adequate.  The RO water would necessarily allow for operation at much higher cycles of concentration thereby improving the energy efficiency of the boiler.  The purer nature of the water may mean that it could still have a lower tendency to scale further improving energy efficiency.  Obviously, the feed water system is more complex and more expensive from a capital perspective and likely carry higher on-going operational and maintenance expense.  Measures may need to be put in place to minimize corrosion potential on the front end of the boiler but that can be far off-set by far less corrosion potential in the condensate return.  In order to determine if the pros might outweigh the cons for a given application, one would have to look very closely at the quality of the makeup water to the pretreatment plant, as well as a detailed water and energy balance from the competing technologies.  Lower gas prices over the past few years have made the hurdle steeper but it's worth investigating.  Each application is unique and will stand on its own merits.