I love rzip but it is important to note that it is not a stream compressor. So it is unsuitable for most of the use cases of LZMA2 which can compress data on the fly or have data appended on the end of a file at a later date.

So rzip is probably excellent for archival situations (not counting any flaws the algorithm may have) but not so great for things like compressed tarballs (which are meant to be appendable).