Much ink has been spilled on how the Crucible is a deux ex machina, and the absurdity of finding it so early in ME3. So I will not rehash that.
I will take a moment to comment on how it changes the tone of the entire game.
The opening tells you the Reapers are an overwhelming force. Shepard spells out how they are superior and it will take everything just to survive.
Then immediately afterwards in the next mission you're told they can be defeated. They were nearly defeated in the last cycle!
All this cycle needs to do is buy enough time to build the Reaper defeating device.
That... is a direct contradiction the game never really overcomes.
The rest of ME3 tries to hammer home how powerful the Reapers are that they're unstoppable. But the rest of the game is also Shepard gathering the resources to build the win button.
There's never a moment of full despair that this cycle can't win. It's if this cycle will be able to win.
Liara even develops a time capsule to ensure they cycle after this one can win if this one fails.
The tension between the deux ex machina and the hopeless war BioWare wants to portray undermines ME3's atmosphere. The Reapers are never undefeatable past the prologue. Just very, very hard to fight.
If BioWare really wanted to portray the might of the Reapers, it would have been more effective to delay Shepard finding the means to defeat them until closer to the end of the game.
Have Shepard win 'small' victories only to see the Reapers immediately overcome them. Have Shepard chase every possible lead to a dead end. Have the final success be a desperate measure that doesn't work. Shepard has to improvise at the last moment. (Maybe Shepard can fail.)
And the whole time, have Shepard bear witness to the destruction the Reapers create. Knowing they have no means to prevent it unless they can spin a victory out of nothing.
But knowing the whole game that you can win... Well, no matter how powerful the Reapers are they're not as fearsome to the player as BioWare keeps telling you they are.