For example, if there is only WHEN NOT MATCHED clause, the base generating expression can be an INNER JOIN
Generally, we should make sure the optimizer is able to work with the plan for Merge statement
Various WHEN clauses can have "extra" predicates. In some cases they may be pushable.
The "source" can be an arbitrary expression - in particular it may include joins which together with the join introduced by Merge itself may need to be reordered.