Not sure if this is an issue for One UI 2, but it was for One UI. It seemed to take 3+ minutes for a chat message to time out and send as a regular SMS when one or more of the party members didn't have good service and the message couldn't get delivered. I think it would be good to have a 30sec timeout for this scenario if the sending phone has yet to receive a "delivered" confirmation. That way you know in most cases that your message has been received within at least 45 secs.
Thank you very much for your suggestion. Your suggestion has been communicated to the developer and the development project manager is reviewing the content. After reviewing your suggestions, we will apply them to the official version if your proposal matches our concept.
