Its future is bright, but that doesn't mean it'll always be smooth sailing.While Amgen's (AMGN -4.16%) shares have underperformed the market over the past 12 months, climbing 19.7% compared to the market's unusually large gain of 35.6%, there's no shortage of reasons to buy its stock. Still, there is a risk that could rain on the biotech's parade a bit over the next couple of years.Let's examine two reasons why the stock is worth buying, and one reason why it makes sense to be just a little bit cautious wit ...