The last time I used UA (*A) miles for a trip between the US and Canada, AC had the best flights available. UA had a few inconveniently timed connecting flights, and US had nothing at all available. (I did not have elite status and was booking a 25,000 mile award.)

UA does discourage using UA miles on other *A airline by requiring you to call them instead of being able to use the web site. And the agents won't tell you about the *A partner award seats; you will have to find out what may be available (e.g. by going to the AC web site) and specifically name the desired flights.