What is the hardest concept to understand in Lojban?

Three candidates.

Lexical aspect: the distinction between achievement, accomplishment, activity and state it took from Vendler. It’s not inherently inscrutable, but rattling off Vendler’s nomenclature is not the way to make people understand it.

The shades of difference between abstractors: nu, du’u, sedu’u, ka, su’u. The distinctions are real, but they are more confusing, and natural languages occult those distinctions behind the matrix predicate class or less granular complement markers.

And the articles. They were confusing before; they got revised; they’re still confusing. They may not be wrong, but they are quite alien.