Apple (NASDAQ:AAPL) has done well by its investors. The company has consistently delivered revenue and profit growth over the long haul, and last quarter, it set a record for quarterly net profit generated by any company. It is hard not to be impressed by what the company has achieved.

As of this writing, the stock is trading just shy of its 52-week and all-time highs. Its market capitalization is about $750 billion, and its price-to-earnings ratio sits at 17.4 times. The company is massive, but one would hardly consider its valuation rich, especially considering that the average price-to-earnings ratio of the companies in the S&P 500 index is 19.8 times.

Nevertheless, even though Apple is a great, well-run company trading at a reasonably cheap valuation, I do not plan to buy shares at these levels -- here is why. 

Apple is basically an iPhone company
It has been widely said that Apple is becoming more and more the "iPhone company," as sales of this device dramatically outgrow its other product categories. In fact, iPad sales declined in 2014, and Ming-Chi Kuo, a prominent (and often correct) analyst, thinks this sales plunge will only accelerate in 2015.

Mac sales are good and growing, but that growth needs to come mostly from market share gains within the premium portion of the PC market, as the overall PC market is basically flat-to-down. Furthermore, while Apple commands much of the profit to be had in PCs, the profit pool does not seem to be anywhere near as large as that of the premium smartphone market.

The company will soon release the Apple Watch, which some analysts believe will bring in significant revenue and profit. However, I remain skeptical that it will be as big as many expect. And now there are the rumors an Apple electric car, and some analysts are already talking about the huge revenue the company could generate if it takes 10% of the U.S. car market.

But, when you get down to it, the iPhone business is the big revenue and profit driver, and I do not think that is going to change anytime soon.

There is real risk here
Apple designs great premium smartphone products, but the competition in smartphones is fierce. Now, I completely understand (and subscribe to) the notion that iOS, the Apple software ecosystem, and its top-notch customer service and retail presence help to build a relatively large and wide moat around Apple products.

However, a solid moat does not promise invulnerability -- there is always a chance that, say, the Samsung Galaxy S6 turns out to be a smash hit and grabs share back from the iPhone 6 series. There is also the risk that as competitors move quickly to adopt new technologies, Apple will be forced to respond by incorporating more expensive components and technologies, potentially hurting its margin structure.

I am not predicting this will absolutely happen, but at the same time I am not willing to dismiss competitive threats to the iPhone. There is risk there, enough that investors probably are unwilling to pay a much richer multiple than exists today. Also, any hint that iPhone sales have hit a near-term peak might lead to a sell-off in Apple stock. 

There is also opportunity
While I do have concerns that increasing iPhone sales could prove difficult after the wildly successful iPhone 6 rollout, it is obviously not impossible. Realistically, companies such as Samsung, LG, and HTC still sell a solid number of high-end phones, so if Apple can gain even more share there (either with the iPhone 6 or with future iPhone products), then that would drive profit growth.

In addition, ARM Holdings projects the high end of the smartphone market will grow at a 4% compound annual rate through 2020. Given the large size of the iPhone revenue base, even if it only boosts iPhone revenue with the market (suggesting no further share gain), then that is still a solid growth opportunity.

I still like Apple and its business, but the risk/reward is not appealing to me
I am a fan of Apple and its products, and my sense is that the company should do well in the long haul. My concern is the current share price might already reflect investor optimism for the opportunities ahead, and that any sign that things aren't going as well as hoped could lead the stock lower. At the same time, barring some massive earnings upside surprise, I do not know how much room exists for the stock to move significantly higher in the near-to-medium term. 

In short, the risk-to-reward balance just does not seem compelling enough for me to purchase shares at this time.

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.