Computing the dependency graph dynamically obviously leads to a lot more flexibility. I'm not really sure what you mean by "reactively" here: if you mean does it reactively update computations as state (and even code, if you want to get Bret Victor about it) changes, then ya.

Data flow by itself isn't necessarily reactive; actually the way it was originally defined to work very lazy push things through. Continuous interpretations where changes were propagated non-lazily didn't come until much later.