I loved the IT for how elegant it was and felt it was a missed opportunity by BioWare. They could have made one of the greatest twists by going beyong the fourth wall, with both the protagonist and the player sharing the same feelings : for those who disliked the endings, both the hero and its puppet master rejecting the reality they were presented and finally finding the truth; for those who the ending, extra free storyline content YAY !
The endings with and without the EC have many problems :
- No boss fight, it felt like a really nice intercourse was shut down before climax. You could argue that the ultimate choice was the climax of the storyline for sure, but it definitely didn't have the intensity to reach the proper climax-like status 
- Rushed decision making : All the other important decisions gave you plenty of time and context elements to be confident with your decision. The last choice introduced crucial last minute details and asked you to make a last decisions just because of that. And that is mainly while the ending betrayed the series, not the actual content.
Forget about the IT for a moment, and picture this ending :
* No Catalyst scene or elevator to Heaven, and no further context elements (destroying technology, mixing DNA)
* TIM/Anderson moment after which you go to the console and dock the Crucible
* (Harbinger boss fight, optional but pretty kick ass nonetheless)
* At which point, you give two choices : Control or Destroy by choosing an option on the console.
* Ending cinematic
And voila, pretty palatable even with the weird atmosphere of the last segment ! Even moreso because the third act from Thessia onward is about the moral dilemma between Control and Destroy.
Aside from synthesis, the other options were pretty coherent with the moral dilemma of the third act and you really didn't need the Catalyst to make your final choice. You didn't even need the information about the Reapers. They didn't explain the Crucible and that was perfectly fine, because it wasn't necessary and I believe it gives a nice "Leap of Faith" feeling to it. Had they not described the Reapers' motives (which completely destroy the myth to be honest because their current purpose was pretty understandable while they were supposed to be "uncomprehensible beings"), they would have kept their intimidating presence.
Besides, you would have a pretty cool feeling of "defeating the unknown with the unfathomable" which would have worked !
- Nonsensical consequences : Control was the decision that made the most sense as it was presented. Destroy gave you new elements to consider (erasing current technology) that were there just to make you consider the other options and didn't make much sense within the framework of the Destroy option. The synthesis option was the most nonsensical, not only because of its premise but also because it goes against the EDI/Joker plot and the Geth/Quarian peace which show that even with the massive differences, these 2 lifeforms can share much. I mean, even if you kill off the Quarians, the Geth are pretty nice towards everyone else !
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I don't blame the BioWare writing team for creating those endings though. They have a track record of amazing storytelling up until that point that I believe the endings as they were are due to external pressure (probably from the publisher's part - EA please stick to publishing and not video game development, please ? ) and probably an excess of zeal : they wanted to give the Catalyst to the fans and pushed it through these constraints and it turned out that the game would have worked a lot better without it.
But it was a real shame and there were so many ways they could have prevented and fixed that. 