Building your own NLP API
Two things to think about are: How are you planning on handling the generation side of things? Entity extraction and classification are going to be useful for the Natural language understanding (NLU) side of things, but generation can be tricky in itself.
Another thing to think about is that the training and development of the pipeline of these models is often a separate problem form the deployment. The fact that you want to use node suggests that you already know about deploying software, I think. But remember that deploying large machine learning models in a pipeline can be complicated, and I suspect that these API's may offer neatly packaged pipelines for you.
I'm building a chatbot and I'm new to NLP.
(api.ai & AlchemyAPI are too expensive for my use case. And wit.ai seems to be buggy and constantly changing at the moment.)
For the NLP experts, how easily can I replicate their services locally?
My vision so far (with node, but open to Python):

entity extraction via StanfordNER
intent via NodeNatural's LogisticRegressionClassifier
training UI with text and validate/invalidate buttons (any prebuilt tools for this?)

Are entities and intents all I'll need for a chatbot? How good will NodeNatural/StanfordNER be compared to NLP-as-a-service? What headaches am I not seeing?