UFC and boxing are completely different. not just in attack but in defence also. There are things fighters do in both sports that would be illegal or stupid to do in the other. For example. Boxers put there heads down or turn there back to avoid getting hit. If they did that in UFC you would receive either a knee to the face or give up a rear naked chock.
In UFC if they were to box the boxer they would not be able to hold legs, block punches by holding, grabbing the back of the head while punching.
We have seen countless times people coming from other sports and trying boxing or MMA and falling short because there not disciplined enough in the areas needed.