I assume it will be as hard as I need to press to force touch on my watch. I'm hoping I'm wrong, as that's pretty darn hard.
You consider that hard?
Have you tried just resting your finger on the screen and seeing how much force it takes for the screen to JUST start to shrink? It's probably mere ounces in pressure required. And very little more pressure is needed to "full depress" it.
With the phone, I'm guessing a little more surface area and more weight will actually make it EASIER to use 3D Toich in iPhone assuming the software engineers haven't built in "more resistance". That's along with hardware engineers....