Thinking about this, it would seem to be a good idea to opto-couple/isolate the sensors before they reach the GPIO pin(s). I could not see any way that the sensors could be connected to a bus so each sensor would seem to need its own 4 wire cable as opposed to a connect to one, then the next and the next type of install. That is a pity as it means a lot more cabling.
Using wireless is OK for plants and beds but is useless on grass with a lawn mower.
Since these sensors provide a variable analog output voltage it would seem possible to be able set a “moisture watering point” for each sensor. This might require some extra hardware to accomplish.
The combination of moisture sensing, water flow control (one flow control, multiple zone measurement) and weather forecasting is a near unbeatable combination for efficient watering.