Immersion rod even though is much easier to operate but they are unreliable. Most immersion rods stop working after a year or two and you have to buy a new one. But in case of a geyser, it can work for many years without any issue.  Also, immersion rods can lead to accidents more often and if they are left turned on without water, they can blast and can injure a person nearby. This is not the case geyser, because most geysers have an inbuilt thermostat that can turn a geyser off if the desired temperature is achieved.