According to this Toronto Star article, it seems AC has been struggling more that other carriers at Pearson. For sure they are the largest, but it sounds like they are having more problems with their staff and contractors than the others.
Air Canada leaves travellers in limbo amid summer flight cancellations: ‘They could have given people a lot more notice’
One traveller can fly to her destination, but the route will no longer be running when she’s due to fly back in just over a week.
“To bring about the level of operational stability we need, with reluctance, we are now making meaningful reductions to our schedule in July and August in order to reduce passenger volumes and flows to a level we believe the air transport system can accommodate,” he wrote.
I get the sense that the "air transport system" might not be main problem.