This ending.... I can only it summarize in one word.
Betrayal! This ending betrays everything that Mass Effect has done! Everything that Mass Effect has set up! Everything that made sense in it's universe! There is just no reason for any writer to come up with these conclusions unless it was a writer who has never played any of the previous games, doesn't know all the details to this particular entry, and was told to just end it on artsy note. It doesn't freaking work.
Remember that stupid, pathos driven kid who was turned into the most important thing on Shepard's mind? He is now revealed as a god like being who is in control of the Reapers. STUPID. He also resides on the Citadel. MORONIC. And his motivation as to why all advanced civilizations must be wiped out? BLOODY OFFENSIVE. What really bothers me about this kid/god being is that they're only exclusive to Mass Effect 3 only. For the whole series to introduce this one element and declare it the most important part of the entire franchise not only short changes the series, but contradicts elements from the previous games! Where was this god like being when ME1 happened? How come it didn't turn the Citadel into the Mass Relay that would bring all the Reapers back from dark space if that was it's plan? And why did it take the form of that stupid, hood jacket wearing kid? There are DOZENS of characters in Mass Effect who are much closer to Shepard who gave up their lives that still inhabit Shepard's conscience. For this kid to take the most important spot is a huge misstep.
And the Reaper motivation? They destroy all advanced forms of life because they don't want them to create synthetics that will one day wipe them out. The civilizations, not the Reapers. That would have made a little more sense, but still be stupid. So these Reapers, who have committed genocide on countless TRILLIONS of life forms over the past millions of years are doing it as a favor to us. This is like that one running gag from "Mars Attacks!" where the translated keeps saying "Don't run! We are your friends!", only this time it's taken seriously here.
Here's another problem with this motivation. Remember the Geth? Some would generalize that they turned on their creators and tried to annihilate them, but that's not true. They merely rebelled and kicked them off their world so that they could be free and safe. They never went set out to hunt down and destroy the Quarians, or any other race. The only time the Geth were ever blood thirsty, murdering rampage machines of death was when.... wait for it.....
SOVEREIGN (A Reaper) INFLUENCED THEM! Did Sovereign at any time show us how his Geth would one day represent what all synthetics are capable of when he took them over? No. Soverign only used them as he used Saren. As tools to further his own plans. So not are the Reapers and this god child killing trillions out of mere speculation, but they're also causing synthetics to do what they specifically don't want them to do! This is like telling someone who owns a gun that it will eventually lead to their death. This is like someone believing that in order to stop someone else from killing themselves with their own gun, he takes the gun, shoots you with it and says "Told you so". See how that logic is flawed?
And the crashed Normandy? First off, everyone who was onboard the Normandy was on Earth fighting the Reapers. Why would Joker land on Earth, take everyone out when there is clearly still fighting to be done (as seen in the cutscenes) and fly pass some anomaly? Why, after crash landing on an unknown planet would they walk out of the wreckage with smiles on their faces? Hey Liara, Kaiden, Joker, Ashley or whoever was left alive on the Normandy, WHAT ABOUT SHEPARD? What about the rest of the galaxy? What about anything?? Did you lose all your memory when you crash landed and all you can think about is how pretty the scenery looks? This part makes no freaking sense at all. It's just thrown in with no set up and no point.