What skills are difficult to master but worthy of mastery?

Once again: That, among other things, would be strong interpersonal communications skills. No matter what your occupation or profession is you will want to know how to speak clearly, intelligently, and use proper grammar when communicating with others. Whether you like it or not, people will judge you negatively if you do not do so. It turns out that Aristotle was more right then he ever knew when over two millennia ago he wrote: “Man is by nature a political/social animal.” (In ancient Greek the same word can mean “political” or “social.” The Greeks didn’t make the distinction we make in modern English.)

Learn to speak and write well. It is much to your advantage to do so.

C’est la vie.