This was my question as well. There are some studies showing that carbon nanotubes can be as dangerous as asbestos, so it seems likely silicon nanowires could be as well:
However, if they can make the nanowires shorter than the minimum dimension of a human cell then it likely wouldn't cause the same sort of cancers as asbestos.
https://www.scientificamerican.com/article/carbon-nanotube-d...
However, if they can make the nanowires shorter than the minimum dimension of a human cell then it likely wouldn't cause the same sort of cancers as asbestos.