To be able to use the basic features the expectation is to have the same basic features as the approach with InitialContext properties has.
This can be very confusing to a user, also as both approaches can be mixed.

Complete wildfly-config.xml client fail
java.lang.UnsupportedOperationException: COM00100: Method "createNewTransaction" of class "org.wildfly.transaction.client.spi.LocalTransactionProvider$1" is not implemented
the UserTransactionObject is -> org.wildfly.transaction.client.LocalUserTransaction

If the URL is passes by InitialContext it works no matter whether the security is set by wildfly-config.xml
the UserTransactionObjec is -> org.wildfly.transaction.client.RemoteUserTransaction