Being a Boeing Company (NYSE:BA) supplier is an uncertain business. Just ask Triumph Group, Inc. (NYSE:TGI). While Boeing has notably outperformed the S&P 500 in the past five years -- driven on by burgeoning aircraft demand -- Triumph Group has languished because of overexposure to disappointing programs. Moreover, the company faces ongoing challenges in 2016. Let's take a look at three of them before you decide to invest any money in the stock.