The actor model is a pretty nice abstraction to reason about completely independent entities that interact purely by exchanging messages. However, for software development, some see the pure actor model as too fine-grained and too restrictive exposing many of the low-level issues such as data races again on a higher level again, and thereby forgoing some of its conceptual benefits.
One of the big questions that came up during my PhD was: ok, now you got your fancy ownership-based metaobject protocol, and you can implement actors, agents, communicating sequential processes, software transactional memory, and many others, but now what? How are you going to use all of these in concert in one application? Finding a satisfying answer is unfortunately far from trivial.