From the research I did before I bought an immersion heater, they are often reported as being unreliable or short lived because they are now made to shutoff permanently if they are ever removed from the liquid before being unplugged.
This safety feature is easily forgotten, and it would be very annoying to render your purchase useless in the middle of a trip. I have thought about how to make a reliable reminder for myself, but all I have come up with is attaching a warning tag to the cord quite close to the heating element.
I think this will work for me, and they take up so much less space in luggage than anything else. I plan to pack it with a stainless steel travel mug, and avoid worries about the cleanliness of coffee makers in hotel rooms. I also bought a Finum filter to make coffee, based on FT recommendations, which has been great to use at home, too.