One note is that symfony implements a "subset" of YAML 1.2, while spyc is YAML 1.0 - I don't know if that affects Translate's usage.
Syck is just bundled fallback. phpyaml is recommended. symfony/yaml is not easy to bundle. See also T235403: Unable to update translatewiki.net: romaricdrigon/metayaml 1.1 requires symfony/yaml ~2.3|~3.0