You've analyzed the writing? The writing of the ending is based on this things :
form of the ending:
-implicit that is here from Mass Effect 1
-paradox that appears in Mass Effect 2 but get more and more developed till the ending. (para = against; doxa = common opinion) the highest level of perception is supposed to be paradoxical because it's beyond our comprehension.
-determinism till the choice that breaks the cycle and make Shepard and the player free of what is imposed by the A.I. and the narration (that's why the original ending was made this way)
theme of the ending:
-the loneliness of the leader (though he isn't alone), and the responsibilities of choices that appears from Mass Effect 1
-the notion of sacrifice that appears in Mass Effect 1
-the concept of religion that is a misinterpretation of events that is implicit from Mass Effect 1
-the relation between civilisation and technology that is the base of Mass Effect 1
-the opposition between organics and synthetics that starts with Mass Effect 1
-the circle and the cycle that appears in Mass Effect 1
-the notion of fate that appears in Mass Effect 1
-the notion of different point of view, level of perception that appears in Mass Effect 1
-determinism
I may have forgotten some things but do you think that a poorly written ending could use so many theme? If you think so them show me a badly written that is as complex as Mass Effect 3 ending.
-The ending was not planned as of ME1
-The ending was not planned as of ME2 either.
-Shepard and the player are still moving down a path determined by the Reapers (in the form of the Catalyst) by the end. Only Shepard is graciously allowed three branches to choose from
-The loneliness is more keenly felt in ME2. In ME3 it sinks to emo territory. Besides which, Shepard is rarely alone on missions in the trilogy. And at the in ME1 and ME2, Shepard's current squadmates could weigh in on critical choices. Such as the Destiny Ascension and the Collector base
-The notion of willing sacrifice is in the trilogy, yes. It's part of self-determination, the ability to lay down one's life for others. Heck we even see it in ME3 with Mordin, Thane, Rila, even Legion. But the endings show Shepard not just sacrificing him/herself, but others too. Shepard sacrifices the freedom and self determination of the galaxy to avoid galactic extinction
That theme actually came up in ME1: "Is submission not preferable to extinction?"
-religion as a theme rarely came up in ME1. It was much more of an allegory in ME2 and ME3: Space Jesus and the Twelve Apostles, etc.
-generally agree about civilkization and technology in ME1. But it faded away afterwards
-organics vs synthetics was a peripheral theme at best throughout the trilogy. Even less once it was revealed the Reapers were partly organic
-cycles yes. Cycles were a big thing. Cycles of destruction vs self-determination
-fate, again yes. Again, self-determination, "There's always another way" Completely abandoned in ME3's ending
-generally agree about points of view and perception. Each game tried to do a Shyamalon-style "twist" towards the end. Backfired spectacularly in ME3
-yes determinism vs free will. Unfortunately by the end "I fight for freedom, mine and everyone's" just leads to SO BE IT!! Then the rocks fall, and everyone dies.
And yes a poorly written ending could do a lot of themes. Poorly. There's a difference between "complexity" and "a tangled snarl"
Other endings that are as bad? Wow, hard to pick. How about The Matrix Revolutions? ME3 kinda reminded me of it.
Professional writer who doesn't read like a professional but like an amateur, so no she doesn't know what she's talking about. It's been proven that she is wrong. What does she know about how Bioware worked on the ending? She says that Bioware didn't planned the ending. It's just like you. She missed a lot of thing in the trilogy and I've said that she admited that she didn't do an analysis! A professional that doesn't act like a professional. Now if you want to find writers who dislike the ending there are two : Melinda Snodgrass and another one but I don't remember his name. They have the same arguments which are totally wrong. You might like them, they think just like you! They dislike Literature, just like you! They write in a populist way! 
No True Scotsman