It's only the excellent character writing that manages to hold things together until the end.
The character writing in ME3 is fanservicy garbage.
Liara - leading the forces of the Shadow Broker and yet still finds the time to follow Shepard around.
Garrus - randomly became a leader among his people.
Tali - also became a leader among her people.
Grunt - became a leader of a mercenary company despite the fact that his ME2 quests firmly established he's nothing more than a follower.
Jack - went from being a mentally ill mess to apparently being fit enough to be a teacher.
The stuff that isn't fanservice is garbage too. Ashley/Kaidan's endless whining, Legion and the geth in general got character assassinated, James.