I'd say that it's not hard to do an acceptable ending, just avoid these ME3 ending mistakes:
1. No Deus Ex Machina solution,
2. No new antagonist (or protagonist? Confused hybrid?) introduced in the last 10 minutes.
3. No violating the Science Fiction genre - everything that happens should have some connection to realism, or pseudo-realism, where it is consistent with how the Mass Effect universe has been known to work, with every new technology or phenomena being such that it can, at least in principle, be given a plausible mechanism by which it happens.
4. An ending concept that does not involve giving a supposedly super-intelligent enemy a silly motivation. If they are super-intelligent, you damn well better make sure that their motivation can survive basic critical scrutiny, without contorted excuses and mental gymnastics.
5. And most importantly: CHECK the ending with fresh eyes - and ROLE-PLAY IT!
Meaning go through the ending from the perspective of the protagonist who doesn't have a god's-eye view of the situation, who doesn't have certainty that he/she is going to win or survive, and who can evaluate the situation only in the light of his/her previous experiences.
A good example of where you clearly failed to do this is in ME3's ending, with the form chosen for the space boy - it's lifted straight out of Shepard's head. If you role-play a Shepard in that situation, confronted with a vision taken from his own memories, what is the obvious conclusion? Given all of Shepard's previous experiences of what happens when you interact with the Reapers for an extended time? INDOCTRINATION. That's what. And the vision taken from Shepard's head proves that the Reapers are in Shepard's head.
Given this situation, you CANNOT expect the player to believe anything but that they are being misled, indoctrinated. Then you fail to give Shepard dialogue options to express this obvious conclusion - probably because you completely missed this conclusion, otherwise you would not have given the space boy the shape you did - and force him to play along. And then inexplicably, for the first time in the Series, the Reapers play it completely straight, and it wasn't indoctrination after all, and you win?
THAT is ultimately what was the worst thing about ME3's ending, that made it inexcusably bad, and a failure of concept and writing.
So avoid these pitfalls, remember basic good story telling, and you'll do fine. I'd settle for fine.