Are going to: a statement about the future
You use are going to if there is a stated or known intention about a future action or event:
"They said they would vote on it today. They are going to make a decision."
It's essentially the same thing as this:
"They will make a decision."
Have yet to: a statement about the present
You can use have yet to in the same situation—but it more accurately reflects a situation in the present:
"They said they would vote on it today—and we're still waiting. They have yet to make a decision."
There is an additional nuance that you are waiting for the thing to happen, rather than it being a simple statement about a future event.
In addition, you can also use have yet to in situations where there is no reasonable assumption about a future event:
"I have yet to win the lottery."
I might never win the lottery, but the use of have yet to is still an accurate description of the present.
(If I instead said, "I am going to win the lottery," as a statement of fact, I could be accused of rigging the event illegally—otherwise, how could I be certain of such a thing?)