I can say the following single line phrase to GH.
Me: "Hey Google, ask Rachio water front yard"
GH: "Sure, here is Rachio"
Different voice: "How many minutes do you want to water"
A two minute timer starts for the front yard zone.
If I start with "Hey Google, ask Rachio to water front yard"
GH: "I don't know how to send messages yet"
I don't think the "ask..." or "tell..." is a must for integration. I have Belkin Wemo outlets integrated with GH. All I have to say is "Hey Google, turn off living room lamp". I don't have to say, "Hey Google, ask Wemo turn off living room lamp".
The IFTTT integration requires some specific applets to be built which is pretty easy to do. That is why I can just say, "Hey Google, water front yard". The slight downside is each applet is configured for a fixed run time (e.g. 5 minutes). I would have to build separate hot phrase applets to do other run times. The new Rachio integration at least prompts me for a verbal run time.
Anyway not trying to fault Rachio here. It has been working very well for me. I was showing off recently using Google Assistant verbal commands on my phone for my sprinkler system guy when he came out to make a repair. The whole home assistant ecosystem is still in its infancy and other GH integrated devices (even Google branded devices) are not exactly 100% there.