First of all, I should mention that I am overall pretty impressed by the Google Assistant on the Shield - I like how it doesn't stop playback of your movie, and controlling the lights and fans in my home without getting up is awesome!
However, there is a MASSIVE problem with the current implementation. When you search within an app, it seems to search only within that app for an answer, which results in you not getting answers most of the time.
Let me give you an example: I was watching Transformers: The Last Knight last night, and I wanted to know who was in the movie. No matter how I asked "Who is the cast of X", "Who was in this movie", etc, it would just search within the VUDU app for a movie title with that question in it!! I had to exit all the way back to the homescreen to be able to search for questions.
For some reason it still let me do home control within the VUDU app without any problems.
Are you serious Google? You can't ask who stars in a movie while watching a movie? That is like one of the top 3 use cases I can think of for Google Assistant on Android TV.
Please correct me if I am wrong somehow.
TLDR; It appears that Google Assistant, by default, searches for answers to questions only within the app you are using.
BONUS: Another disappointing aspect of assistant is that the movie database is not up to date. If you do a voice search to watch Transformers The Last Knight, it will only show it is available on Google Play Movies, which is not true. How long does it take to update the database?
I noticed this, as well, while in the Youtube app.
I asked "who is playing in the football game tonight?", and it came back with youtube search results. That's not ideal.