Well, about the pain, you can choose between when you have overtrained and have your muscles rigid and locked, or when you go barefoot and the floor is very cold and your toes begin to curl by themselves. None of them pleasant experiences, I swear.
About nails, my thought is that them, as well as the skin, are parts of the body. Why can't they transform like the bones, muscles and internal organs do? I understand the visual efectiveness of that in the older movies, but not getting them transformed is plainly "unrealistic".
And finally: keep up with the feet TFs, in the mainstream they are always the big forgotten ones, even though they can be used very effectively when you have to cope with censorship and need to show some TF on screen
.