Welp, my browser ate my longer response. But in short, when I was at WhatsApp, dist worked fine over WANs, I think the WA contributed pg addresses the issues we saw with pg2. It was obviously not ideal, or normal, but sometimes our WAN connections would get bandwidth limited due to congestion or other issues, and things would generally keep going without unexpected problems: replication got behind when the change stream was more than available bandwidth and there's no separation of dist traffic, so if replication has minutes or hours of delay, so does any other traffic in that direction, so interactive traffic will timeout, net_adm ping will take minutes or hours, etc. I guess technically the connection ticks must not quite suffer the same queuing, because the connections would not be severed by tick timeouts.
The other issue related to WAN that we'd see is when you start a mnesia node, it loads all of the tables from peers if peers have copies of the tables, but out of the box it doesn't have a concept of LAN vs WAN peers, so it may load your half a terrabyte or whatever worth of data from WAN instead of LAN. Loading all that data from LAN might be undesirable too, but that's another story; you could probably maybe come up with a way to store and later replicate changelogs for offline mnesia peers, but that doesn't come out of the box, and WA didn't build it either.
The community at large often reported maximum dist cluster sizes that were way less than what we ran in production, and comments about WAN distribution that didn't match our experience either. shrug It does help to have a very solid private network between nodes though. Dist in general, but mnesia specifically, don't like for nodes to disappear. pg2 has some quirks too, but pg resolves them, afaik, as I mentioned earlier. Some of pg2's issues were rooted in global locks, you've got to be careful about those if node lists are rapidly changing and there's contention on the locks.