@NumericField works by encoding the value in multiple fields, so considering there is a well known limitation that the field must be unique this doesn't surprise me too much.
The current faceting implementation relies on the field caches, and we already populate the field caches while applying some tricks to decode types so I guess this is not hooked up to the faceting engine.. but we need to see if this is still possible when switching to Lucene's new Faceting module.

I need to move this to Hibernate Search >=4.3 as it depends on the upcoming faceting work; will create a separate issue to document the limitation in 4.2.