in the current API. write concern are managed by only one class, beside the fact that several roles has been identified (https://docs.mongodb.com/manual/reference/write-concern/)

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The consequence is that it leads to lack of consistency states for write concern objects.
Inconsistency managed by the driver. But in this case it's hard to cover all unexpected configuration. And If it happens, then the user faces an exception followed by a potential core dump.

Furthermore, the documentation of the API should explicitely provide which state to avoid ...

All that can be solved by building consistent states. And this can be obtained with a clean classes hierarchy that declare a class for each identified role.