Most common fault with ferroresonant devices is that resonating capacitor going low value, which results in poor regulation, though you also will have to be aware the voltage across that capacitor is very much higher than mains, and is dependent on load current. Capacitor 10% low in value change it, and preferably with a 440VAC rated motor run capacitor over the 370VAC rated one, as it will last longer there. Have seen them burn those capacitors out, with the converter working fine just with lots of noise and smoke. Otherwise the diodes failing, though normally they are pretty reliable, as the limiting results in a very long conduction angle as the secondary coil saturates and presents a almost square wave output, so the diode conducts over a larger portion of the sine wave. that and inherent voltage limiting within limits, so the lead acid batteries get to float voltage and stay there.