For many years, Apple (NASDAQ:AAPL) was widely viewed as a "can't lose" investment. The stock handily outpaced the major indices for years as its fundamental business performance continued to dazzle.

Image source: Apple. 

Indeed, the company struck virtual gold with its iPhone product line, revolutionizing the mobile phone market and, indeed, the computing market as we know it. And investors who got in early could have made fortunes. 

However, over the last couple of years, Apple has transformed from a hot growth stock to a mature, megacap technology stock that pays a solid dividend and routinely shrinks its share count through stock buybacks.

For many risk-averse investors, a proven company like Apple is exactly the kind of stock that they're looking for. That said, even a highly profitable industry leader like Apple carries investment risks. Let's take a closer look at a couple of those risks.

Apple relies so much on a single product category

In Apple's most recent fiscal year, it generated approximately $136.7 billion in revenue from sales of its iPhone product line out of a company total of $215.639 billion. That's more than 63% of the company's revenue base coming from a single product line.

There are all kinds of risks associated with having this much exposure to the smartphone market.

For one thing, the market itself has cooled off quite a bit. Market research company IDC predicts that the smartphone market will grow at a 5% compounded annual growth rate from 2015 through 2020. The smartphone market no longer seems to be a high-growth one.

Next, Apple has been having a hard time even growing with the market. In fiscal year 2016, Apple saw its iPhone unit sales drop even in a market that expanded, implying market share loss. There are many potential explanations for why Apple's iPhone business performed so poorly that year, but suffice it to say, if this market share loss continues, it could be tough on Apple's financial performance longer-term.

Finding growth elsewhere is tough

Apple has certainly been investing in trying to grow out non-iPhone businesses. It has put a lot of effort into its iPad line of tablet computers, it introduced the Apple Watch line of smartwatches not too long ago, and it has been investing heavily in building out its services business with products like Apple Music, Apple Pay, as well as continued growth from its App Store.

In fiscal year 2016, Apple saw growth from its Other Products (Apple says this "includes sales of Apple TV, Apple Watch, Beats products, iPod and Apple-branded third party accessories") and its services (Apple explains that this "includes revenue from Internet Services, AppleCare, Apple Pay, licensing and other services").

However, its iPhone, iPad, and Mac businesses all declined in that fiscal year.

Apple is certainly onto something with its services business, and there's a chance that Apple Watch could be a long-term hit, but finding significant, sustained growth outside of its core iPhone/Mac/iPad businesses isn't likely to be easy. 

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.