Over 10 million people are playing the markets using Robinhood, the red-hot stock trading app that was offering no-commission stock trades long before the major online brokerages followed suit. Some of the stocks Robinhood users have been drawn to this year have been downright awful, not too surprising given the inexperience of Robinhood's user base.

Robinhood users have also piled into higher-quality stocks that wouldn't be out of place in a typical retirement portfolio. The list of the 100 most popular Robinhood stocks is dotted with blue-chip companies as well as some broad-based exchange-traded funds.

One of the most popular Robinhood stocks is Apple (NASDAQ:AAPL). On the surface, this seems like a reasonable pick. Apple's iPhones are a cash cow, accounting for around 40% of the U.S. smartphone market, according to Counterpoint Research. Once someone becomes an iPhone user, switching costs make moving to an Android-based device a pain. And all those iPhone users are potential customers for Apple's growing list of services.

It makes sense that Apple is a popular stock on Robinhood. However, anyone who thinks Apple stock is a safe bet that can't go wrong could be in for a big surprise. Here are three reasons why Apple may not be the best stock to own.

A risk dial turned to maximum.

Image source: Getty Images.

The iPhone is not a growth business

The smartphone market in developed countries is largely saturated. There just aren't many potential new customers who don't yet use a smartphone. This means that Apple's iPhone business in these countries is largely just selling existing iPhone users on new versions.

That's all well and good, but it's not going to be a major growth driver for Apple. Apple's iPhone business is dependent on the company convincing users that its latest devices are the next big thing. That gets harder as smartphones get better. Five years ago, the year-to-year leap in smartphone quality was significant. Today, not so much.

Yes, Apple can grow its iPhone user base in countries like India where it doesn't dominate the market. But those markets are brutally competitive. Apple currently has a single-digit market share in that country, and inexpensive iPhone alternatives from Chinese manufacturers will make it tough for the situation to change.

The iPhone business will ebb and flow, with some year's models proving more popular than others. But it isn't going to be a meaningful source of growth for Apple.

Antitrust lawsuits could kill the services growth story

Apple has been making a big push into services in recent years, selling its user base on subscriptions for music, video streaming, games, and cloud storage. The company even recently announced an all-in-one subscription that bundles many of its services together.

Services have become big business for Apple. In fiscal 2020, Apple generated nearly $54 billion from its services segment. Services are now the second largest source of revenue for Apple, topped only by the iPhone.

While the services growth story sounds great, there are a couple problems. For one, Apple generates a huge chunk of its services revenue from a deal with Alphabet's Google that makes Google the default search engine on iOS. This deal, which involves Google paying Apple between $8 billion and $12 billion annually, is being targeted in an antitrust lawsuit against Google.

These payments are essentially all profit for Apple, so losing them would take a big bite out of the bottom line. A separate antitrust lawsuit against Apple in the U.S. could be coming as well, focused on the company's policies and fees related to the App Store and payments. If Apple is forced to reduce how much it charges developers or loosen its grip on app distribution or payments, the services segment will suffer.

The services growth story for Apple could hit a brick wall if things don't go the company's way on the antitrust front, and earnings could plunge under the worst-case scenario.

An optimistic valuation

Despite a largely stagnant iPhone business and a services business that's under attack by antitrust regulators, Apple stock is priced for growth. Apple currently trades for around 36 times earnings; that's down from closer to 40 times earnings a couple months ago, but it's higher than at any other point since the financial crisis.

Apple's net income dropped 7.4% in its fiscal fourth quarter, dragged down by a slumping iPhone business. Net income was up just 3.9% in fiscal 2020, despite strong demand for Mac computers, iPads, and other non-iPhone gadgets amid the pandemic. Those aren't the kinds of growth rates that scream "36 times earnings."

Apple is already worth around $2 trillion. The pricey stock has provided exceptional returns for shareholders up to this point, but the company may be too big and facing too many risks to do anything but disappoint investors from here on out.

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.