When setting up zones, the choice of shrubs is limited to one choice, which sets a root depth of 18 inches. This would be fine for a medium-to-large, established shrub. With smaller shrubs, this might lead to under-watering, especially with flex-scheduling.
I found this to be the case when I started with flex scheduling and found that one zone had no scheduled waterings. When I looked into it, I found that adjusting the root depth for my Nandinas to 8-9 inches gave what I thought was a more appropriate watering schedule. As a somewhat experienced user (as most early adopters will tend to be), I was able to adjust this appropriately.
As majority users come on the scene, I can see more potential need to make this clearer. Most people will probably feel that a shrub is a shrub is a shrub.
Perhaps a setting for small/medium/large shrubs with varying root depth?