Is the Hertzsprung-Russell diagram still a valid instrument in studying stars, given that it is over 100 years old

Is the Hertzsprung-Russell diagram still a valid instrument in studying stars, given that it is over 100 years old

Please answer in at least five sentences each.

Is the Hertzsprung-Russell diagram still a valid instrument in studying stars, given that it is over 100 years old?

What are some of the challenges that you might face if you wanted to observe and study stars from Earth?