Unlike the Outlook+Exchange combination Sieve is an actual open standard (RFC 5228 & RFC 5804 & co) though that can be implemented by all mail servers and mail clients. Or is the Outlook+Exchange feature based on the same?

Looking at the sieve website it seems quite a few desktop and web mail clients do have support for it either out of the box or as an add-on. So I would say that having this functionality is more on the "common" side than "borderline unique".