A game is only as good as how it ends... Unfortunately ME3 ended horribly.
Now, when you finally get to the end of this long journey, imagine every choice you made and everything you've done becoming completely irreverent. Remember helping the Quarians get their home world back?.. Doesn't matter. They're now trapped on Earth. How bout ending the conflict between the Turians and Krogan with the hope of helping their respective planets. It was pointless. They're now trapped on Earth too. And what about all your attempts at galactic peace and stability? Won't happen. The mass relays are destroyed so galactic civilization will crumbled.
Quite literally in the span of 15 minutes, everything that you worked towards across all 3 games no longer matters. At the end, the game sticks 3 final decisions in front of you and forces you to choose. Not only are these endings completely unaffected by your choices throughout the series, but all of them are 90% the same with only slight variations. The only thing all your hard work determines is whether or not your Shepard lives or dies.
All in all, the ending is full of plot holes and leaves you with more questions than answers, as well as a feeling of helplessness and frustration. No matter your choice, the mass relays are destroyed, and your crew is stranded on an unknown planet far from Shepard's location. This game was the end of Shepard's story, and when ending the journey of a character, especially one as well known and loved as Commander Shepard, the most important thing to do is give the audience closure. This game failed to do that, for both Shepard and his crew.