If you need to press down harder on icons to use 3D Touch, you'd need more leverage on the phone. But I think the iPhone 6s Plus' unweidly size will start to bother some people. Take for example that camera icon at the top right corner. Not only do you have to wiggle the 6s Plus in your hand to reach that icon, but now you have to press down hard on it. I just feel 3D Touch made the larger iPhone just a little bit more difficult to use.