At the moment we are recommending either powerline adapters or the TP-LINK WiFi adapter. I agree that build-in WiFi would be a better solution, but it’s not really cost-effective. The TP-LINk adapter is $20 and powerline adapter is $30. These are not that more expensive than USB WiFi dongles. Also, powerline adapters are more reliable and less prone to WiFi signal issues.
As another part of the added cost: having built-in WiFi means the product needs to be FCC certified (OpenSprinkler is FCC verified, and does not require certification because it doesn’t have intentional radiators). Having intentional radiators means we must test the product to acquire an FCC ID, even if the circuit is made of all FCC-approved parts.