I think when the question is: to know or not to know, and that is way more important than the remote chance that an aspiration might let loose some cells. These cells are going to metastasize on their own without the need for a FNB to be the catalyst to do so, so in my opinion to not do it for fear of spreading something is holding you in the "we don't know what it is" place unnecessarily, and letting a potentially dangerous situation go undiagnosed.

FNB can be hit and miss. But someone that does lots of them is going to punch the needle in about 5-7 places, ensuring that a representative collection of cells is made. It sounds worse than it is, afterall it is a "fine" needle.