Multiple boxes create an overly complicated system. Wiring, power usage, mounting, maintenance and utilization are multiplied and become more costly.
Also, coordinating timing and usage between the boxes and zones would also be a nightmare for those of us who make seasonal and other adjustments throughout the year.
The whole idea of a sprinkler system is to centralize control and usage while lessening the watering effort. Multiple boxes would not do that.
With smart WiFi boxes, it seems like there ought to be an easy way for expansion of programs and station zones within the unit. In my opinion, every control box should have an ability to expand to at least 5 programs and 25 station zones to allow for large, as well as individualized, watering areas for greatest flexibility and efficiency.