ORP does not reliably correspond to FC level. Contrary to a paper put out by a person working for a company that sells ORP systems, FC level is the best indicator of sanitation, not ORP. There are many different things that can affect the ORP reading, only some of which correspond to the FC level. In a commercial pool you can get a professional operator to re-calibrate the relationship daily, so it can be worthwhile in that context, but for home use that is the exact opposite of what you want.
There are far more reliable and less expensive system that are readily available and work just as well. ORP requires frequent maintaince and expensive probe replacement, when simple percentage or timer based systems work just as well in residential pools.
ORP systems have known issues with SWG based systems that cause the combination of ORP and SWG to to completely and utterly not work a significant percentage of the time. SWGs add dissolved hydrogen gas to the water when they are active. Dissolved hydrogen gas pushes the ORP level down fairly rapidly, so you often get the exact opposite reaction from what is intended. In pools with low hydrogen outgassing rates, this results in the ORP system calling for constant chlorine addition for hours or days at a time with no correspondence to the FC level.
ORP systems require fairly low CYA levels, which result in significantly increased chlorine consumption, and thus higher costs.
ORP systems require very tight PH regulation. This requires additional equipment and management which would not otherwise be required.