It is typically less performant than an equality comparison (it is usually faster to search for X than "anything except X").
There are always other ways to write queries , there is rarely exactly one way.
Sometimes the DB2 optimizer rewrites the queries dynamically. You can use the db2 explain facilities to view the access-plan and the actual query (after any rewrite).
Actual run time performance of your query will depend on many factors (db2-version, operating-system that runs the DB2 server, query optimization level and registry settings, indexes and other performance structures, accurate recent statistics for tables and indexes, bufferpool sizes, cardinalities and many other factors).
This make the query a little neater and easier to maintain if you have to make alterations in the future, such as adding more values to the list.
p.s. I think you may want an index that has at minimum the leading column of ASSIGNEDTO.