By extending the table length, you are increasing the amount of turns it takes for foot CC units to reach combat. This means they take more damage coming in and have less time to hit back once they reach enemy lines. In effect, you are giving shooty armies an unfair advantage because they have an extra turn or two to do exactly what they are good at. At the same time, you are hurting CC armies because you are reducing the amount of turns they have to be effective.
Besides, how many IG and SW players agree to having MORE terrain than the recommended 25%