Ok, first, regarding what people were talking about with the carrier providing you with a wireless access point/router to have wifi calling better - what they're doing is they've setup QoS (quality of service) on the WAP, so the wifi calling traffic gets priority over everything else on your network (as well as your connection to your ISP). It's also possible that, if your cellular carrier is your ISP, they could prioritize the wifi calling traffic further down the line, throughout their network.
The fact that there are different carrier priorities for Wifi Calling are actually pushed down via the carrier settings. Some people get the option to pick what the order is, some don't (I don't with my carrier...I can turn wifi calling on or off, that's it). I'm at home with 4 bars of cell signal, yet wifi calling is enabled.
How calls can be dropped on wifi calling is simple....while most of us have very good broadband internet at home these days, there's still the possibility of traffic problems on the internet or even your own network. Even if you have quality of service setup, there's outages and traffic congestion on the internet. The phones are very good today at switching your call back over to cellular if it can detect that the signal is being lost, but sometimes it just happens too quickly for the phone to do anything about it.