Really comes down to shitty writing more than probably time constraints then I guess, but yeah I hear ya.
Mostly, yes, but you also have to look at what fans probably expected from the final entry of a trilogy, based on BioWare's promises and marketing of the franchise. I think most people didn't just want something well written, they expected to see the fruits of their labour come to fruition. All their choices, decisions, and actions representing
something in the final game.
Bad writing can factor BioWare's idiocy in specifically negating these choices. Not ignoring them, but rendering them pointless. The actual content that is there, the lack of any substance and closure: that is bad, dumb writing of which there can be no excuse. Bad planning can also factor into this.
But think about the hypothetical best the ending could be. I think a lot of people wanted slides, or scenes, reflecting on all their decisions. Doing this isn't as simple as writing such things. It has to be
built. It requires people making assets, and recording voice where necessary. Scripting, cinema, testing, and so on. Staff required to do this would be unable to work on other aspects of the project. This would also have to be budgeted. And, creating all these scenes would factor into just how long the game would take to make.
Now, if EA didn't really give a shit about any of this, and prioritised getting an 'entry point' Mass Effect 3 out the door, how do you think they'd respond to BioWare hypothetically saying "we want more time to make more endings"? It's not their vision of the project, and expanding the ending might delay the game a month or two past the target date.
After the game had already been delayed once.
There's a whole bunch of variables involved in this. I firmly believe awful writing is the number one factor, but I don't doubt that publisher meddling and poor planning also contributed.