Walt Mossberg has a funny "series" on his Twitter where he screencaps all the blatent failures he has with Siri. Many times, he or a follower will attach a screenshot of Assistant completing the action. It's not scientific by any means, but it's fun to go through and provides some concrete examples. His biggest issue seems to be that Siri is unreliable - there are queries that work, but only sometimes, and that makes the experience of using the product terrible.