/W/ without /w/?

I have a question about a recently started conlang: Is there any  universal I
don't know about that prevents a language from having the  unvoiced
labial-velar approximant, /W/, without it's voiced counterpart, /w/? or  /m_0/ without
/m/? I would like to add these features, but wish to  know how 'realistic'
they are. Thanks. Dan.

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