Here is my take on concurrent zones, specifically utilizing Flex Daily…if anyone cares.
Flex is an awesome, dynamic, always changing schedule, and if Rachio allowed multiple zones to run (thru programing), some days you might have 1 zone run, another day 2 zones, and another day 3 zones depending on how the fluid schedule ran that day. Issue I see is, when I combined 3 of my grass zones (thru wiring), I had to adjust the spray patterns of the nozzles to account for the change in pressure thru the system. Combining the zones actually brought my zone pressure into the sweet spot for my particular nozzles, so it worked out well for me. But, if you are dialed in with 1 zone running, and Rachio decided to run 2 or 3 at the same time, you can have a drastic drop in pressure and volume, thus reducing the PR of your sprinklers and making the Rachio algorithms inaccurate. I know it sounds miniscule, but depending on the GPM of a zone, it could make a significant difference in how the zone performs.
Just my $.02.