"If China won't let US companies operate in China, the US shouldn't let Chinese companies operate in the US." Sounds like some middle school arguments, not some serious thoughts
Yes, it's kind of difficult to define the 'hardest' problem. Some issue I considered 'hard' is more because of my missing relevant knowledge in this field rather than the nature of the issue. And of course, I don't want to say "I spent a few hours to learn something to get things done, which is just changing a property value".