ME3 is definitely not just "the bad one." ME1's story was pretty sloppy and ME2 had almost no story at all. And guess what? Mac Walters wasn't lead writer on either of those. It was Drew. His story treatment for ME3 was also a sloppy mess, so all Mac Walters really did was fail to improve on what Drew already came up with.
Series was doomed from the start.
It wasn't doomed, it had problems of course but they could have been fixed. They werent with a lot of them just rolled over to the next games.
But yeah Drew's "dark energy" thing was in my opinion worse than reapers reaping advanced species to prevent synthetic domination. The real thing that messed up the me3 ending was the starbrat and the 3 choices.
The problem as I see it was that Drew had absolutely no idea where he was going with the whole Reaper thing. Or it seemed like there was no endplan. The idea was cool in the first game and it worked, the whole indoctrination thing, semi-organic ships that have a mind.
Everything up to and including ME2 and all the dlc end made sense, sure it was not bulletproof and had its fair share of mistakes but there was no "this makes absolutely no sense".
And even majority of ME3 still made sense, still everything fitted more or less. I liked Leviathan DLC, it seemed in my opinion a classic scifi storyline.
Anyway I am convinced that a lot of problems on technical and implementation side could have been fixed by extending the development time. As for the ending, it could have been saved if they didn't go for unique(which it wasn't check Deus ex) and "impactful" but instead relied on what they knew and could do well, the kind of ending that you got in DAO.