The issue with Chat function for customer service is how many simultaneous chats the agent is expected to work. If they are handling one or two at a time, it can work OK. But bean-counters want them to handle more, and then it gets frustrating and not effective. And they also tend to want off-shore / ESL agents to handle them, as there is no voice accent issue, which is OK if the agent has the necessary language skills. But Chat tends to bring out acronyms and internet slang, which can be a struggle.
It could be a great niche for in-flight assistance though.
If United is rolling this out, it is due to some perceived cost savings for them and not as a customer service enhancement.