Hi William,
Japanese batrachologists (why do people shun this nice word?!) may have a different view on things, but the 1988 publication
Studies on Chinese Salamanders (Ermi Zhao, Yaoming Jiang, Qixiong Hu, Yuhua Yang) offers this phylogeny of the seven genera of Hynobiidae based on morphological character analysis and comparisons:
...as opposed to these earlier ones:
So you can see from this that
Onychodactylus it is considered more derivative than some other genera.
Onychodactylus is indeed the only member of Hynobiidae that lacks lungs, but
Liua and
Batrachuperus also have cornified coverings (on their palms, soles, fingers and toes). Still, only
Onychodacuylus has what is described in the report as "sharp claw-like coverings"
This report has a very nicely detailed comparison of the hynobiid genera, aside from what's mentioned above.