Only Iceborn is a problem. You nerf that and Sunfire will go back to how it was. Nerf Sunfire only and it will just be replaced by another item.
I finally got to go against a tank Fizz today. He's literally always banned, so I've had no experience facing him up until now. He's completely annoying, but if it wasn't for the sticking power given by Iceborn I would be able to handle him incredibly easily. For reference I played Garen.
That said Iceborn and Sunfire, in their current states, should be fine items. They do their jobs for the champions they were designed for, and are not overly strong in those cases. It's not like anyone can build Iceborn and an applicable health/resistance item and do well, it's only the assassins.
That is why assassins are getting changes that will hopefully make them build properly.