For this connection, should be no need to go through screening in YYZ so Nexus doesn't enter in to the picture.
This itinerary is a little tight but ought to be just fine. If AC sold it, they're going to do their best to make it happen... passengers and luggage.
If some travellers choose to have three or four hours between connections like this, that's entirely up to them. Why not four or five or six? We all know there are things that happen that can delay us. It's kind of like.... I booked a 1200 flight on disembarkation day, is that too early? Then you get the responses; OK, unless there is fog or OK unless the ship hits a rock or OK unless immigration takes hours to do something that normally takes 15 minutes. You should change your flight to 2200 or next day just in case.
Jeepers... just plan something and go with it.
I guess I've demonstrated my opinion on this; no doubt there are alternate views.