There are basically 2 types of kettles/water heaters, the ones that have some sort of safety feature/thermostat that auto switch off and the ones that don't. which one is better depends on your needs and how you plan to use it. For the average traveler in areas with relatively safe tap water, a kettle that has the auto-shut off feature when the water comes to a boil is perfect. If you fall asleep or are absent minded this is a really nice thing to have.
For those that travel in areas where they have to rely on the kettle to boil the water long enough so that it can be safe to drink, kettles without the auto shut-off feature is better. The reason there are such a large number of negative reviews for these kettles without an auto-shut-off feature is that the person using it most likely forgot about it or left it boiling too long. For this type of kettle, it's best to use some sort of timer to remind yourself when the water has boiled long enough and to avoid the kettle overheating/boiling dry.
As for immersion heaters, most have an auto-off safety switch built into them. The trick here is to unplug the heater and wait a few seconds before taking the heater out of the water. If you take the heater out before unplugging, or take it out too soon after unplugging, the simple sensor will cause the heater to automatically shut-off. As there is no reset switch in something this inexpensive and simple, you've just killed your immersion heater and will need to get a new one.
For short trips and if I'm traveling solo, then my immersion heater is usually stashed somewhere in my bag along with cup or travel mug. For longer trips, if I'm traveling with a buddy or if having hot water is essential to my day then I bring a small dual-voltage travel kettle such as the Kenwood dual-voltage kettle, the
Austin House dual-voltage kettle or even my Sanyo dual-voltage cooker. Overall my favourites in terms of ease of use are my small travel kettles mentioned above. The Sanyo cooker is nice when you need to cook something as well and don't want any leftover flavours making their way into your tea.
A note on purchasing kettles and other dual-voltage electronics online: most items have a European or British plug and may or may not come with a small set of international plug adapters. The Austin House kettle mentioned above and usually anything from Japan or Taiwan have US-style 2-prong plugs.