SCADA systems are much less reliable than PLCs and DCSs. This is because of the following reasons:

• SCADA systems are updated quite frequently to add or change tags, units of measure, scripts, alarm thresholds, and synoptic. This means that SCADA applications have to be restarted often. They usually run on Windows operating systems, so they are also frequently rebooted to apply security patches or other installation tasks.
• SCADA systems are built on several modules that execute different tasks. Sometimes, these might be overloaded and they may drop the communication tasks that are in charge of feeding the I-IoT data stream. 
• Connecting directly to a SCADA system implies that we are able to talk to it according to its API or SDK. Although there are some SCADA systems, such as InTouch or iFix, that are very common on the market, in any case we have to develop, maintain, and update the connectors for the SCADA system. Just like for the PLC, this require a considerable effort, considering that each SCADA system is liable to require enhancement and fixing, so you have to deal with the compatibility and the backward-compatibility of the developed connectors. Again, similar to the PLC, another option for using SCADA systems as a data source for the I-IoT data flow is to connect to it through its OPC server. The considerations about OPC interfacing that were mentioned in the section on the PLC are also valid here.