When the developers fail to deliver an interesting game to play, that is when they fail. Luckily ME3 is a fantastic game to play but they could have put so much more variety and interesting setpieces to make it better and you can easily make the story fit around these moments. As for immersion, this game has it, but so does something like Dark Souls and that is super video gamey. They are not exclusive.
Plus in an RPG it is kind of hard to not draw people out of the world when you have to navigate a bunch of menus to advance stats and stuff. If you guys really wanted true immersion you would argue to remove all that as well.
Gigantic video game bosses that take away from the story and usually also take away from the gameplay are just lazy crutches.
Just like laugh tracks in shitty network comedies.
Cliche'd stories in movies that are recycled over and over.
Actors who basically play themselves.
Anytime a portion of a game/movie/show is doing nothing beyond HEY IM A X SEE IM A X!!! It's basically always a failure.
Why did the Reapers attack Sanctuary again?
Didn't want cerberus to be able to steal their troops through cerberus indoctrination based on reaper tech.