The framework-method parse_osmfile defaults to xml when reading from "-", but Osmium::Output::OSM::create defaults to pbf. In my opinion, Osmium::Output::OSM::create should default to xml, too, as the pbf writer is not yet implemented.
additionally it should be possible to choose the input/output format using a second parameter on both methods, to allow reading and writing arbitrary formats.
The framework-method parse_osmfile defaults to xml when reading from "-", but Osmium::Output::OSM::create defaults to pbf. In my opinion, Osmium::Output::OSM::create should default to xml, too, as the pbf writer is not yet implemented.