We have tested our agents with all of the Dialogflow integrations.
We learned a few things.
1) Not all the integrations seem to support Rich Content. This will likely change over time but right now you sort of have to know which integration is on the other end of the conversations. For Example,the Browser integration does not support images/cards. When you get the request you need to check on the integration before sending anything.
We ended up doing a bunch of handling on in the Fufillment scripts.
2) Most are fairly straight forward – Once you get the hang of what it wants. The two that we found were different is Alexa and Contana. Alexa integration you can take some JSON over to the Alexa world but not much more. Contana was never completed.
3) Support is very good. Dialog flow can take a bit to get back to you but they will get back you with usually a quality answer.
4) Google Hangouts was more difficult than we expected. If you already are setup and use the G-Suite it might be easier for you. We had to setup G-suite and then try to integrate which is not the usual case.
Good Luck.
DF Assistants.