I use the factory classes quite extensively to guess the correct implementation of a file that needs to be unpacked. The current doc does list that for lzma and 7zip files, the auto detect will not work. I have worked around this by looking at the file extension, and hope that its correct.
For .Z files, I can only uncompress the file if I explicitly tell the factory that its using .Z compression, the auto detect never works. I'm using 1.7, but I dont think its fixed in 1.8 either (after looking at the bug fix list).
Either its a bug in the doc, or in the auto detect of the compressor factory.