You're going off of 2 Avatars...among hundreds of reincarnations. I don't think that's fair. Besides, Korra has both strengths and weaknesses. Aang couldn't bend more than air, but he also had a much easier time accessing the spirit world. Korra, on the other hand, was good at the physical side side, being a bending prodigy, but has almost no connection to the spirit world.
Have you also considered the fact that Korra's personality is nothing like that of a typical waterbender? She's all fight and fire.
When Iroh explains it in Bitter Work: Water is the element of change. The people of the Water Tribes are capable of adapting to many things. They have a sense of community and love that holds them together through anything. They also are more defensive fighters, turning their opponent's strength against them by control.
So far, Korra's style is nothing like that. She prefers to attack then defend. On the other hand, air is free and light, without resistance and no direct attacks.
Also, look at Roku. He was too indecisive during his lifetime, and showed mercy to Sozin...he could have prevented the war, but he decided to spare his friend.
Iroh: Fire is the element of power. The people of the Fire Nation have desire and will, and the energy and drive to achieve what they want.
Water is change, and Roku couldn't adapt when he heard Sozin's plan to invade the rest of the world. He wasn't willing to accept his old friend was different until it was too late.
As for Aang, he struggled with earth because it was the opposite of his personality.
Iroh: Air is the element of freedom. The Air Nomads detached themselves from worldly concerns, and they found peace and freedom.
On the other hand,
Iroh: Earth is the element of substance. The people of the Earth Kingdom are diverse and strong. They are persistent and enduring.
Aang is peaceful and optimistic, the complete opposite of solid, determined earthbenders.