Merlins do produce a reasonable amount of water, but their efficiency is not very good.
So they produce lots of waste (expensive if you are on a meter) with a efficiency of about 90%. A local fellow w/c has just replaced his and with softish water (90ppm) his pure is 10ppm. So he still needs to put it through a di for final polishing off to 0ppm.
Other Roman type R/O's do produce water slower but with an efficiency of between 97 - 99% using American sourced membranes. The Korean/Chinese copies are cheaper but not as efficient.
If we want 0ppm then we all use resin, but obviously, the higher the TDS is, the more resin you will use over a period of time. Your resin di vessel will be after your Merlin and a seperate item.
Spruce