if u r talking abt the built in controller, e.g. Red Sea Aquazone Plus; it does really pose a problem. the flaw is that the controller cannot be calibrated, there is simply no such function with the Red Sea unit.
good news is the Aquazone Plus unit that i've got is already abt 1.5yrs old with the same old original probe (i am the 2nd owner). juz calibrated the probe a couple of wks back with 475mV calibration solution. the resulting reading from the calibration is 468mV. a deviation of only 7mV. not too bad for a unit/probe that's 1.5 yr old. but i can't say the same for RAV-65, results varies.
if u r gonna connect the ozoniser to ur skimmer intake, do make sure ur skimmer, airline, etc r made of ozoneproof material. use carbon to remove excess ozone, etc.
as for orp controller, u can check out the milwaukee instruments SMS510 or SMS 125.