We used CHAT ($9pp for whole cruise) on the Luminosa over the NYE cruise and agree with those reports. It was hit and miss. My messages to my teen sometimes got delayed and even not arrived at all. I think it depends where you are and if you are in your cabin or moving around. Just keep checking that the WiFi is working where you are. I liked having it.
I was able to use some OBC to pre-book a Noumea Shore Excursion for our NYE Luminosa cruise. I then went back in to my booking and checked and it had removed the exact $ amount from the OBC that was left.