I guess the things I don't like about Chat are the same things I don't like about pair (or team) programming. I've always thought of programming as a solitary activity. You visualize the data structures, algorithms, data paths, calling flow and stack, and so on, in your mind, with very high throughput "discussions" happening entirely in your brain. Your brain is high bandwidth, low latency. Effortlessly and instantly move things around and visualize them. Figure everything out. Finally, when it's correct, you send it to the slow output device (your fingers).
The minute you have to discuss those things with someone else, your bandwidth decreases by orders of magnitude and now you have to put words to these things and describe them, and physically type them in or vocalize them. Then your counterpart has to input them through his eyes and ears, process that, and re-output his thoughts to you. Slow, slow, slow, and prone to error and specificity problems as you translate technical concepts to English and back.
Chat as a UX interface is similarly slow and poorly specific. It has all the shortcomings of discussing your idea with a human and really no upside besides the dictionary-like recall.