When it is justified to use UA OPC and UA OPC architectures over MQTT|OPC UA Standard|Forum|OPC Foundation

It is not really an either or question since Part 14 of the OPC UA specification describes how use to use OPC UA with MQTT.

Note that MQTT provides a way to deliver unstructured binary data. MQTT requires that all parties to know in advance what will be in the messages which means that if you want interoperability between applications developed by different vendors at different times you need a standard like OPC UA which provides a rich framework for describing data.

See this link for more:

https://opcconnect.opcfoundati…..mqtt-amqp/

I suspect the question you really wanted to ask is when should you use disconnected message broker communication and when should you use point to point client-server communication?

Broker based communication makes the most sense when you have loosely couple components where messages can be sent without requiring a response. If you are designing components where each message must be confirmed (i.e. writes to update device state) then client-server makes more sense. Many real systems will have a need for both modes depending on what needs to be done. e.g. UA client-server is used to configure a server to send predefined datasets containing device state changes via MQTT. Once configuration is complete the server will only send MQTT messages.