One possible complication you may encounter with the ESP8266's ADC is that on the NodeMCU boards, the first reserved (RSV) pin that is right next to the ADC pin is directly connected to the ESP8266 module's ADC input. This can potentially cause issues with electrostatic discharge ESD exposure. Essentially the discharging of a high voltage, but low current, into the MCU. Adding a small capacitor on this RSV pin to ground can help reduce this risk.
One thing that this system obviously cannot help with is to keep your plants pest-free. This means that though the watering may be automated, that doesn't mean that you can just ignore the plants. The regular checking of the plants for any issues, as well as the system for any issues that may be developing (disconnected tubing, things that have fallen over due to cats, and so on) remains an important task.
