Those are actually easy. The ones using the first versions of React with custom state management and weird conventions however, are a different story.

Those, and debugging jQuery UI problems.

I digress but in my career of using every technology possible, the pain I had debugging a huge (HUGE) jQuery UI project could perhaps only be matched by trying to make NHibernate (Hibernate ORM but for .NET) behave in some complicated (but CRUD after all) data-heavy projects.