I think their problem is that it's not an ending. It tells you nothing and it ends nothing.
You walk away knowing less about the ME universe than you would if you never played the games.
Not the way you should end a trilogy.
Some people think the unknown is philosophical, inspiring, and artistic, while others think it's cheap, cliche, and annoying.
Who is wrong or right probably isn't the question. The real question is that how you want to end a game like Mass Effect with so many imported choices and characters, and customers who want to actually know how that universe ends.