Hi, I would say that 'data' is now what is called a "collective noun" in English. Note for instance, the use of the word HAIR. When we say she has red hair, indeed yes, we know that she does not have a single strand of hair, but we are referring to the millions of hairs, but we do use the singular "FORM" to express the conceptual category of PLURALITY, and accordingly the verb it takes is also for the singular form.
So, going by that logic, DATA as a word is perhaps now in the collective, the form is plural (as opposed to datum), the usage in conceptual terms can be singular or plural (does it even matter?) and the sentence treats it as singular.
So, the "data is complex" should suffice... :-)