The actual users are a different matter. I certainly see that
they could be quite legitimately discomfited if the mapping is one
that they do not expect. They might also find unusable a server
which simply rejected any names with a double-s character with
NFS4ERR_BADCHAR, which would be compliant with the spec, but
not very useful for a German speaker.
The problem would be if a user have an NFS server which do handle
double-s. Then they switch vendor to another NFS server which claim to
be compliant with the spec. They move the files there (not using NFS)
and suddenly some files are not accessible from the same client.