Back in the day, Mac Walters was known on the Bioware forums as Mac "The Hack" Walters. They guy was responsible for the low points of the story in ME3.
Intro mission on Earth? That was Mac.
Kai Leng? That was Mac.
Thessia Mission? That was Mac.
Mars? Mac.
Final mission on Earth? Mac.
The endings? Mac and Casey.
The guy was a good character writer, as he's mainly responsible for Garrus and Liara, so credit where it's due. But as writer for an overall narrative, and a leader? He sucked fucking dick.
ME2 and 3 had good writing
Andromeda however was written and produced by spastics.
Too bad.
ME3 certainly had good writing, particularly with the Rannoch and Tuchanka storylines which writer Patrick Weekes was mainly responsible for. But overall? No ME3 did not have good writing. It wasn't just the endings. Right from the start of the game the writing went to the fucking toilet. When Commander Shepard of all people needs to be told that we can't fight the Reapers conventionally on Earth, and he needs to escape and go try to unite the Galaxy, something has gone very wrong with the writing. This guy knows better than any other person in the galaxy how powerful the Reapers were, the countless cycles of civilizations that were destroyed, and his answer to the problem was "We fight or we die!"
I remember when I first played ME3, red flag after red flag after red flag was raising in my mind during that intro. The difference in the quality of the writing between ME1 and ME3 was like watching a delicious T-Bone steak slowly rot and get moldy over time.