This unsubscribed me from twenty+ communities on all of my instances. It first tried to subscribe all of my accounts to all of the communities of my largest instance, which is what I wanted it to do. One instance failed to sub to 20+ communities, which was fine, as it was a smaller instance.
Ten minutes later, however, I got a notification that it had unsubscribed all of my other instances from those communities as well. It took me about an hour to figure out most of the ones that had been lost, and even now I think I’m missing a few.