To those of you who may be wondering why Korra didn't try stopping the arm...
…because I saw this complaint about the scene, and because it had even been bugging me, I finally figured out the reason why:
It’s not that simple.
Look at how Korra stops the mech.
She’s not throwing simple little precision blasts of water up at the thing and fusing its joints together, she is sending raging torrents of water on top of it and freezing them.
There was one little gap left as an opening, and Asami managed to navigate the hummingbird down and land on that exact spot.
And that was all Korra was going to be able to give her.
If Korra had tried freezing the arm, odds were that she would have just frozen Asami and Hiroshi to the side of the mech. There was literally nothing left for Korra to do but stand there and watch, waiting for the two of them to detach as the machine began to break out of the ice. Even if, say, she had been catching the ice and refreezing it right back where it was, it wouldn’t have worked—Kuvira’s machine was too powerful, and once she had enough power sent to the arm, it would have broken through, anyway.