The team I was teaching struggled with the concept of streams and reactivity. It's kind of like programming backwards, especially if you're used to an imperative mindset. They just had a hard time with the concept of a stream.
They also had a tough time remembering all the operators in the library.
Hmm, I suppose I can see your point. When doing reactive things you have to think "what should this depend on". When doing imperative things you have to think "what depends on this that I need to remember to update". I always found reactive programming to be easier to reason about generally, but I suppose if someone's stuck in an imperative mindset it can be difficult to pick up reactivity.
They also had a tough time remembering all the operators in the library.