David Benoit walks through how to configure a Polybase cluster:
When you get to the PolyBase configuration screen, you have the option to run this as a standalone instance, or to use the SQL Server as part of a scale-out group. Personally, I don’t think that anyone should ever choose the standalone instance option. You can always run a SQL Server configured for a scale-out group as a standalone instance, BUT you can’t change (at least not today) a SQL Server configured as a standalone PolyBase instance to run as part of a PolyBase cluster once you have completed the install.
This note sounds like the argument for clustering all SQL Server instances.