The Unarchiver seems to have a problem with the Amiga .lzx archives that are using the highest possible compression. The Unarchiver refuses to uncompress such archives. Archives compressed with a lower compression unpack well with The Unarchiver.

I've tested this by making such problematic archives using an Amiga Emulator (I've came across this problem while checking some old archives from my real Amiga 1200) using the last amiga version of lzx that came out:
http://xavprods.free.fr/lzx/
http://aminet.net/package/util/arc/lzx121r1

I was only able to unpack the mentioned archives on my mac by compiling the unlzx source code (from the lzx page or the wiki page below).
http://en.wikipedia.org/wiki/LZX_(algorithm)