The order to consider adapters in, and why the generic one is a LAST RESORT rather than a shortcut.
FIRST: THE APPLICATION ADAPTER for that system, where one exists — the Fusion adapter, the adapter for a named SaaS product. It knows the system's catalogue, generates the structures for you, handles its authentication and its pagination, and understands its events. Using it is an afternoon; rebuilding that by hand is a week and it will be missing something.
SECOND: THE TECHNOLOGY ADAPTER for the protocol — FTP, database, JMS. Where there is no application adapter but there is a well-defined transport, this is the right level: it handles the mechanics without pretending to understand the business object.
LAST: REST OR SOAP AS A GENERIC CALL. Anything can be reached this way, which is exactly why it is tempting and exactly why it is last.
WHY THE GENERIC ONE IS A LAST RESORT. Everything the application adapter did for you, you now do by hand: the structures, the authentication, the paging, the error semantics, the version differences. It works on the day you build it and it is yours to maintain forever — and when the target changes, nothing tells you except a failure.
THE TEMPTATION IS REAL because the generic route is FASTER FOR THE FIRST CALL. An application adapter takes twenty minutes to configure and REST takes five. The difference reverses by the third operation, and it reverses permanently at the first upgrade of the system on the other end.