Undertow server needs default-host to be set to be able to be used as Undertow subsystem default-server otherwise Wildfly would not start due to:

ERROR [org.jboss.as.controller] (Controller Boot Thread) WFLYCTL0362: Capabilities required by resource '/subsystem=undertow' are not available:
    org.wildfly.undertow.host.test-server.default-host; Possible registration points for this capability: 
		/subsystem=undertow/server=*/host=*

This is dangerous as after calling a reload operation, the server simply fails and crashes. One can accidentaly kill the server with this and !manual! fix is necessary to the configuration to be able to start server again.

I think that there should be some check that configuration of the newly created server is complete and valid before it is set as a default server.