It has been three years since Drupal 8 is released. Even Drupal community has providing seamless support through forum and other resources(eg: IRC) still people are struggling during D8 development. So I'm sharing my D8 development experience here.
Majority will agree that most difficult part in Drupal development is theming. As Drupal is rendering html with its own containers and attributes achieving the exact design provided by UI developers in Drupal feels like a challenge to the developers.