comparing opc ua with MQTT is comparing apples with oranges! OPC UA is legacy protocol designed by non IT professionals from OT background, inspired by propitiatory COM/DECOM of Microsoft later migrated to XML then UA adapting to new IT technologies as it evolved to SOA and webservices and survived as a specification! Its time to think of a more suitable and simple alternatives like WOOPSA and PubNub etc which are lighter and easier to understand and implement !
Similarly people with IT backgroung try to apply all their knowledge of Enterprise applications like JAVA/EJB/Servlets/SQL/NOSQL etc to Automation which is an overkill! What is required is an optimum balance!