At the time of writing, it is not known whether $search (either executing in the classic engine or in SBE) will work with clustered collections. This ticket will track the work to determine whether this is the case, and to add test coverage for it (regardless of whether it works or not). At a minimum, the two engines should have the same behavior.

Indeed, in true bean counting fashion, if it isn't supported, supporting it could be deferred to another ticket.