Being loyal to the source material is the least of the problems of some adaptations. It not following the lore or the plot or even the characterizations exactly is one thing, but even on their own, most of those movies were terrible.
Following the source material exactly doesn't really determine whether or not a movie is good.
Most of those movies were made by Uwe Boll, the problem of those movies was not that they were videogame adaptations, the reason they sucked does not apply to the ME-movie.
Ok, in cases like Tomb Raider there was not enough in the games to be a good movie, so adding/altering stuff not in the games is a necessity, but that is not the case with Mass Effect.
Nevertheless you need to include the defining aspects of the game (=be loyal), otherwise you will most likely end up with another Super Mario Bros. Movie.
To put it in a catchphrase: There is no Mass Effect without Shepard, the people around him/her and the Reapers.
The "first contact war" (and other "great" ideas from players on this forum) whould result in 90 minutes of humans and turians trying to kill each other, with other species only present at the end, that is something for expanded universe (like the comics) and fanfiction.