Wait, really? I didn't know this...I did get an incoming call notification on my iPad the other day though, after enabling wifi calling (I'm on AT&T). Everything seemed to be working fine. I'll check my Mac to see if my call came through there as well, but I don't see why it shouldn't since it went through on my iPad.
Continuity still works on my phone when there is only Wi-Fi Calling available. But I cannot assure that it would always work with all carriers, since the feature might be optional.