Those water heaters will do what they claim, but it's what they don't tell you that will give you an unpleasant surprise if not careful.
If you take a shower while using one, first getting wet and then turning off the water at the head while soaping up, you will get hit with very hot water when you open up the shower head again. That is because, while stopped, the water was held in the hot coils and gets very hot as it had nowhere else to go. If that water is held on the coils for even a minute or so, you will feel like you are getting cooked for a while when you open the shower head again to rinse off. You can reduce the problem by remembering to direct the head away from you until the water is cooler again.
Stopping the water may be fine when getting hot water to wash dishes or if the water is constantly running while washing a pet, but not good if it will be hitting you after heating on the coils for a while. Also, if you leave the shower run while soaping up to avoid the problem, you will use up more of your water supply and also fill your gray water tank sooner.
I use solar
water heating bags as much as possible for shower hot water, but use a portable Zodi heater, very similiar in concept to the ones you listed, to heat water when the days are cold, cloudy, or I wasn't able to set out the bags early enough to heat the water if driving for a long period of time that day.
I will take some pics of the system I use to avoid the overheating problem and will put them in a new thread so they don't get confused with the tankless water heater thread here.