It seems there are some key differences between working with Tableau in MySQL and via the API in FileMaker.
I was unable to find a list of what to expect so, in case it helps someone, here are the differences I have found so far (in no particular order):
1. A Tableau Workbook must be tied to a single FileMaker layout, rather than multiple data tables.
2. The FileMaker admin must put thought and planning into which fields are presented on the layout to ensure the required data, and nothing extra, is extracted to Tableau.
3. The FM Admin should create different FileMaker layouts for different Tableau Workbooks.
4. Related fields can be included on the layout.
5. The current found set displayed in the Tableau layout is ignored by Tableau.
6. Tableau will extract all data from all fields on the layout, except...
7. Tableau ignores fields in portals on the layout.
8. Key fields from join tables do not need to be included on the layout.
9. Loading data from FileMaker is significantly slower than loading from MySQL.
10. FileMaker data can only be extracted and is never live.
11. The Tableau user has no options to change or build relationships.
If I have of this incorrect, please correct. Also, please add to this list as appropriate.