My wife just returned home after a week visiting relatives out of state. Following a week with me I am pleased to report her house plants as well as the two dogs are all alive and well. I remembered to water the plants and dogs as well as feed the dogs.
Seriously, those watering systems really are not very good for all the reasons mentioned. The best solution is a timer system where all the plants receive water daily at the same time for a short period of time. Timers like this can be had very inexpensive and generally have a battery backup to maintain the date and time information. A water supply, a solenoid valve and a drip line is about all that is needed. Soil moisture sensing is a pain in the butt and with the posted circuit your probes will corrode and go away or as said disappear. Your plants will become very unhappy.
Ron