The British vote to leave the European Union could make things complicated for BMW's U.K.-based Mini and Rolls-Royce brands. Shown is BMW's Mini factory in Oxford. Image source: BMW.

What: Shares of German luxury-car maker BMW (OTC:BAMXF) were hit hard on Friday. As of 11:30 a.m. EDT, the company's American depositary receipts were down 10.24%, to $76.30.

So what: Concerns over the British vote to leave the European Union hit many stocks hard on Friday. But BMW was hit harder than some rivals for a couple of reasons.

First, BMW owns the Mini and Rolls-Royce car brands, both of which do their manufacturing in the United Kingdom. Simply put, Brexit could complicate the export of those vehicles to the rest of the Eurozone, as well BMW's ability to import parts for those cars from the European mainland. 

Second, Britain is a significant market for BMW -- and obviously, the rest of Europe is a hugely important territory for the German automaker. The potential for trade barriers between Europe and the U.K. are only one concern. Another: A significant recession in Europe now looks more likely than it did a day ago. That could hit BMW's bottom line hard at a moment when it's scrambling to ramp up spending on advanced technology in order to keep pace with global luxury-car rivals.

Now what: As with most of the stocks hit hard on Friday, the real story is uncertainty: Investors see risk arising from Brexit, but right now it's vague and hard to quantify. It may be a while before we clearly understand the real risks to BMW's balance sheet and bottom line. In the meantime, I urge BMW investors to heed my colleague Morgan Housel's advice and sit tight for now.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.