I see GameDev.net being more of a engine and language agnostic knowledge base, so if GameDev.net started to create forums for specific engines, there's a good chance that we'd have to create forums for CryEngine, LumberYard, Havok, etc, and I imagine a few of those would have very little activity. Anyways, that's my $0.02 and I don't run this site, so I leave this as a consideration for those who do and for the community here
This is sort of how I see it as well. Already, the API forum isn't super-duper active, and further fragmenting it by splitting off Unity and UE forums is probably a waste. Any engine-specific questions are probably fine in API, and anything else is probably fine in Game or General, IMO.
Welcome to the new members/refugees.