What are the advantages and disadvantages of using old thermostat?

The old Mercury style thermostats provide a wide range from "on-off" and can provide major energy savings for hot water/steam radiator heating. These heat systems have wide swings of heating and the newer electronic thermostats cannot be adjusted for the wide swings. The largest manufactuer of thermostats in the US, Honeywell, acknowledges this but even Honeywell determined that not enough homes use the older heating systems to be practical to make an electronic unit for their application.