One could argue either, but I would choose "complexity".
Student or Learner
In the following sentence:
The vocabulary has none of the .......... and ambiguities of a natural language, so why has Esperanto not thrived?
The correct answer is complexity.
Could it also be complexities? That's what I wrote.
But what makes you choose "complexity" over "complexities"? Is there a rule in this situations?
I'd choose "complexities" in that sentence too - either "complexities and ambiguities" or "complexity and ambiguity". It doesn't matter which.
I can see a language having more than one instance of ambiguity. So a countable "ambiguities" works.
On the other hand, if it is complex, it is complex. It does not have one complexity here and another complexity over there. It is complex, therefore it has complexity.