ME3 is just a bad game, and not only for the ending.
Exactly this. Even as a standalone it comes off as a cheap Uncharted knockoff that does have some moments of narrative weight and okay-ish combat. I still think, compared to its inspirations like Gears of War and the like Mass Effect 3 still only goes so far with it. It has serviceable cover and shooting mechanics only, not fantastic, even though I like the visceral melee variations (That also feel very hardlocked) and the things that makes it get away with it are tactical pausing, squad commands and tech/biotic abilities.
But there's a lack of depth to both encounter design and squad leveling I think. I think the way the RPG skill trees were expanded were a token gesture to those that complained ME2 was too streamlined. In hindsight I consider ME2's skill tree perfectly succinct; giving you just the skill strategies and variation you need that actually seem to have an impact. I didn't ever
feel the impact of choosing "+0,5 biotic force vs 12% shield recharge" in ME3 and things like that. There's too much focus on raw gameplay skill in shooting, covering and tactics in ME3 to warrant that much of a skill tree. ME1 is the only game in which the RPG systems are there to determine how succesful and prepared you are for combat.
And with the downgrades to game design, writing and player agency ME3 just stood out as an actual poor game to me. A 6/10. I remember there was a column in my danish GameReactor magazine to the review roundup of the game, the main review gave it a perfect 10/10 saying bullshit like "They did the impossible and made a perfect conclusion to a trilogy." but the column on the right had a shortened but more critical review complaining about the story's nonsense, fallacies and lack of depth to game-design and concluded on a 6/10. That was cathartic to me as someone who expected everything like ME2 and
then some but got a sorry excuse for a sequel instead.
It's right down there with Dead Space 2 vs Dead Space 3 for me. The propotional loss of quality is the same.