http://www.computerworld.com/article/3138045/android/google-assistant-clarity-consistency.html
Summary: Assistant is inconsistent across devices - different assistants stitched together across apps, each behaves differently and responds to commands with varying degrees.
http://www.computerworld.com/article/3129098/artificial-intelligence/why-google-a-i-is-the-last-user-interface.html#tk.drr_mlt
Summary: Google assistant is context-aware and offers true AI that can remember and recall facts about the user for personalization, but connecting to other 3rd party assistants drops off the experience out of AI framework. 3rd party assistants (Siri, Alexa, Cortona, etc) require users to memorize tasks - Alexa offers 3,000 and allows for add-ons by developers. Assistant can figure out what the user wants.
Samsung acquired 'Viv labs' which was founded by the original Siri team when Apple bought Siri.
http://www.computerworld.com/article/3132396/personal-technology/google-pixel-phone-packs-a-powerful-voice-assistant-but-it-s-no-alexa.html#tk.drr_mlt
Summary: The assistant on Pixel is good at context awareness, but its task list is limited - delivery of commands is sub-par compared to Alexa or Siri. Smart understanding, not strong on for example, search results.