Google Assistant and the Harmony Hub used to work perfectly for me. Now a simple thing such as, “Hey Google, change the channel to 7” results in several different audio responses, none of which is changing the channel on device the TV is using.
I have never seen a product regress in such a way before. Google, you should be embarrassed.
Sorry to hear about the frustrations that you're having with the Google Assistant, I'm happy to help.
I would suggest visiting our Google Assistant Forum and writing to them there as this is a question that they would be able to help answer.
Please let me know if you have any further questions.