Just gave Nightmare a run. Loving the atmosphere now, this is how Hardmode should have been, no HUD. The only thing I find that might be troublesome is the total removal of the minimap, even when you find those map boards its still blank, and you may need to recall your previous playthrough to navigate through some parts, or actively remember where the areas are. Its still an enjoyable experience for a second playthrough, with the Alien now even more actively hunting you and you only have what's on you for getting around. A lovely challenge.