@giwayume: Actually it works without "-out" argument, umodel is using contents of umodel.cfg for various parameters, including save packages path and keep structure option (if you've saved something via GUI once, you should have that config). With that config set and placed near umodel, some command line parameters are read from there.
Alternatively, you can use specific build from
this topic - it can now use "-out" argument for saving packages into particular folder as well, and also keep structure option is exposed via "-nokeep" argument.
Sample command:
umodel -save -nokeep -path="Path to paks" -out="Path to saved assets" asset_name