I was doing auto water changes for about a year. Worked great... until it didn't.
I had the system set to remove water from my refugeum, 1st compartment of my sump, and add it to my return pump compartment. I'd estimate that very little 'fresh' water was being lost.
Oh... my DOS pump, the one pumping new salt water into the tank, stopped. The one pumping water out kept right on dosing so. Auto top off did what it is supposed to do, and refilled with fresh water. I didn't notice until my corals started looking bad... salinity was 1.012. I highly recommend a Salinity probe with an auto water change system.