Drupal 8 is released soon and it's a great improvement, but still has some technical debt. There has been improvement in it, but there is plenty of legacy legacy left in the code base:
Projects with large codebases, such as WordPress or Drupal, are always going to have more technical debt than small projects such as Silex or Twig. However, large projects usually have lots of contributing developers and thus, technical debt could be more easily assessed on large projects.
Read more from this blog post: Technical debt of the most relevant PHP projects