I've flown through YYZ plenty of times. As noted, the worst I've ever encountered there was just how mind-blowingly busy it can be, and the consequences of that. Which is... well, that happens at a lot of airports.

My concern was that the specific things I asked about at YUL, as far as the security lines, were specific detailed complaints I had seen made by passengers. The departure-time effect on the security line, for example. I've never seen that happen at YYZ, but I have seen staff there make plenty of token attempts to pull people to the front of the check-in lines based on departure time (often fruitlessly, since there was no way those passengers would make it through preclearance and security in time).

The idea that people wouldn't even be permitted to enter the security line -- which, again, was a specific complaint I saw multiple people make -- would be an entirely different level of problem. Hence I asked about that.

Again, I'm just pointing out concerns raised from my own poking around in various forums. The complaints are out there, and I don't feel it's unreasonable, when I see that kind of thing, to turn to what I consider a reputable source (people on FT, who are likely to have tons of experience with airports/airlines) to find out what bits are true.