midiphony-panda Spine has some custom import code (to create SkeletonDataAssets, Materials, etc...) and it's maybe not recommended or tested with Unity's parallel importing ?
Thanks for asking. Regarding the spine-unity import logic it is safe to enable parallel importing.
In case anyone should ever encounter issues with parallel importing (even though we have not yet encountered any issues ourselves in tests, nor heard of any), please do let us know, we're happy to fix them.