How long does it take the German KMK to evaluate a degree and what are the criteria?
I can't comment on the wait times, but the goal of the evaluation is to ensure that your degree is comparable to the equivalent degrees offered by German universities. They will be looking at things like the number of credits in your major, and if you have training in the field sufficient to be equal to the German counterparts. The grades will also be considered.

The odds of "failing" are impossible to judge, but if you're short on credits or have a low grade-point average, you are likely to run into trouble.