Hello,
the various SubscribeItem/SubscribeMultipleItems differ only in the fact that some offer "shortcut" way to specify the full set of parameters, so really there is no performance or functional difference in the outcome, so you are free to choose anything that best suites your input parameters.
Using the Callback, or just event handlers, is probably the only relevant decision to be made here, so you have asked the right question.
I would use the Callback if you have completely different processing algorithm for different items (or sets of items). Using the callback makes it straightforward to associate the processing code with the items. If you used event handler in such case, you would probably end up with some kind of big 'switch' statement at the core of the event handler, just to differentiate between the incoming item notifications, which only complicates things.
Best regards,

Hello,
Short answer: Most likely, this has to be resolved with the OPC server vendor. It may even be a correct behavior. The fact that other OPC client does not see the isse may be due to timing, and other factors.
Details: The Quality values are never generated or modified inside the EasyOPC component. They simply pass through. And, the OPC client side has no way of "mismanaging" the OPC server to generate improper qualities.
Some OPC Server provide Bad quality as first notification each time a new OPC item is added, and only then start supplying the Good quality values. That is correct behavior as well, and may be behind what you are observing.
If you are using repeated reads (not subscriptions), EasyOPC could, in background, move items between OPC groups, and in such case the Bad quality will appear not only at startup, but also later. This should not happen if you are using subscriptions, though. Some old version of EasyOPC could do it with subscriptions as well; are you sure that your version is indeed 5.0.1377.1?