That example is just a lot of different things going on, but nothing itself is very complicated:
Trigger sets a key in the player model:
SetKey("controlmode","auto")
Trigger sets the camera model's target to the player. The camera script follows the target, when one exists.
Trigger sends a message to a sound entity to play the music.
Player moves to his target until he reaches it, and some more events are triggered.
...and so on.
A bullet hitting a player, hurting them, and updating the owner's score was about the most complicated single interaction I could think of. Any other ideas? One advantage of this system is it encourages you to create a defined set of actions with the messages, instead of making spaghetti code.
One disadvantage is when you have something like the roads, and you want to tell if a wheel of a car lies on the road. You need to communicate with the road script, but how do you get that info back? That is where it gets sticky.