The current Alexa/Harmony integration offers more features than Google Home/Harmony: specifically the ability to easily control volume, pause, etc. With Google home (which I had for a week), you can do that well with Chromecast, but not with Harmony. 
Since the granular skill (Harmony Red) has been released for echo, We can hope it will eventually arrive for Google home, but it hasn’t yet.
Right now the Google Home/Harmony integration, whether it’s through SmartThings or through IFTTT, can only deal with stuff at the activity level.
It is possible to still control volume that way, that’s how we used to do it with echo, but it’s clunky and a little confusing.
The first thing you have to do is create Harmony activities that will do exactly what you want. Once you have those working (so that all the things stay on that you want to have stay on, etc.) then it’s pretty easy to use a virtual switch to integrate it. It’s getting the activity set up in the first place which is complicated.
For one thing, harmony will not let you repeat an activity, so if you want to turn the volume up by three and then turn it up by three again you have to have two different volume activities and go back-and-forth between them. And then you have to return to the original activity so you get control over it again.
If all that sounds like a pain, it is. 
But the brand-new harmony red Skill for echo is great and avoids all that nonsense, so to be honest probably the best thing to do would just be to wait for that same functionality to come to google home.
If for whatever reason you really need this functionality now with Google Home, see the following thread. Although the voice control examples given are for echo, it will work the same way with Google home once you get to that point. The main thing to pay attention to is how the harmony activities are set up.