I mostly used RiF, but installed the official app during a trip last year to do logistics with someone who was using the built in chat.
I don't tend to always have data on my phone, because WiFi is everywhere, and just add some when I'm going on a trip like this. I'd loaded up a fiver's worth the morning we left, which usually sees me through an entire month in these situations.
Woke up after the first night away to an alert that my data was all used up. And on further investigation, guess which app was the culprit? Just literally sitting there overnight while I was asleep, gobbling up a month's worth of data to do god knows what.
I uninstalled the app.