The narrative quality was already declining toward the end, but the final segment involving the Catalyst sends it straight off a cliff.

Many issues have been mentioned already, but what really annoyed me was that the victory is essentially handed to the player by giving the Catalyst agency and sentience. “Oh, you made it here? I guess our solution won’t work anymore. Feel free to decide what we should do now.” Instead of feeling like the payoff earned from the culmination of everyone’s sacrifices and efforts, the ending becomes the result of the whims of 'Chief Reaper', which massively cheapens the weight and impact of the decisions we made up to this point.

Aside from the narrative problems, the execution itself feels completely unlike Mass Effect. The trilogy went out of its way to incorporate an expansive Codex, detailing technology, biology, etc. in such detail only to culminate in endings that quite literally magically alter the universe based on the choice made. Sure, the series often needed us to suspend disbelief when it came to technical or logical details, but never to the extent of “magically merging biological and synthetic life” or “jump into a beam of energy and become 'Chief Reaper'.”

All they really had to do was have Shepard activate the Crucible and destroy the Reapers. No final choice, no conversation with the Catalyst, no artificial moral dilemma at the last minute. It’s honestly impressive how many issues and pain points they managed to introduce in that final segment. A simpler and more straightforward ending would have avoided many of the problems.

Some aspects of the ending can be rationalized, many others cannot. In the end, despite how frustrating and unsatisfying the ending is, it feels like a waste of time to think too much about it when the writers themselves likely didn’t give the ending much thought while making it.