Pros
When I was applying to Canonical, some of the negative reviews here and on other company review sites were admittedly concerning to me, so I wanted to come back here after almost a full year of employment to note that my experience has been entirely positive. As someone passionate about operating system software development, my role at Canonical is nothing short of a dream role, complete with work that I enjoy, major opportunity for upward growth, teammates that are a pleasure to work with, and very fun travel and career development opportunities. Having a day-to-day experience like this that contributes directly to the world's most widely used operating system for servers is extremely rewarding as well, since I know that my work has a major impact on the global compute infrastructure. If you are a prospective applicant, specifically one who is passionate about advancing Linux and open source software in a big way, Canonical is full of incredible opportunities for you. I'm extremely glad I was not swayed away from this opportunity by concerning reviews. My advice would be to pay attention to your experience with the interview process - for me, despite everything I had heard about it, it consisted exclusively of very interesting conversations about topics I am passionate about with people I was excited by the process of working with - which is precisely the experience that I have had since joining.
Cons
Some growing pains due to time it takes to hire appropriate candidates