Despite selling different products, Fitbit (NYSE:FIT) and GoPro (NASDAQ:GPRO) have been sold off together since the beginning of the year. Both stocks are now down about 40% in 2016 on fears that their lack of product diversification and moats could cause sales to plummet. GoPro's fourth-quarter sales warning on Jan. 13 also caused Fitbit shares to fall nearly 5% the following day.

Image source: Company websites.

Fitbit bulls will argue that it's silly to sell the stock on GoPro's woes, especially when holiday sales were seemingly sweet for Fitbit but sour for GoPro. But Fitbit bears will highlight the troublesome similarities between both companies.

Let's discuss four of GoPro's numbers that should worry Fitbit investors.

1. GoPro's sales growth in 2013: 87%
When GoPro filed for its IPO in May 2014, it reported that its 2013 sales surged 87% annually. Last February, it reported that 2014 sales had slowed to 41% growth, but no one seemed too worried. But earlier this month, GoPro warned that its 2015 sales would only rise 15%. Analysts now expect that figure to flatten to 0.6% in 2016 due to the commoditization of its core action camera market.

Fitbit's growth trajectory looks similar. Its revenue grew 255% in 2013 and 178% in 2014, but analysts expect that growth to have slowed to 144% in 2015 and rise just 33% in 2016. That big decline in sales will likely be caused by smartwatches with health tracking features and cheaper basic fitness trackers flooding the market. Between the third quarters of 2014 and 2015, Fitbit's market share in the global wearables market fell from 32.8% to 22.2%, according to IDC.

2. Total number of GoPro devices: 6
Last year, GoPro diversified its product line with three new cameras -- the Hero 4 Session (originally $400, now $200), the Hero+ LCD ($300), and the Hero+ ($200) -- which doubled its portfolio to six devices priced between $130 to $500. But as seen with GoPro's preliminary 2015 sales, that scattergun strategy didn't boost sales to acceptable levels.

Fitbit has been doing the same thing. In late 2014 and early 2015, it launched three "premium" devices -- the Surge, Charge, and Charge HR -- which offered more smartwatch-like features than its basic Flex, One, and Zip activity trackers. To Fitbit's credit, that shift was fairly effective, since 79% of its sales came from those three premium devices last quarter.

Fitbit Blaze. Image source: Fitbit.

But in early January, Fitbit launched the Blaze, a $200 smartwatch which lacked a developed app ecosystem and wasn't competitively priced against similar devices. Those weaknesses caused some analysts to predict that the Blaze would flop. Launching the Blaze also indicates that Fitbit doesn't have much faith that the Surge, Charge, and Charge HR can keep competing against smartwatches. As a result, Fitbit is using the same clumsy scattergun strategy that sank GoPro last year, and the Blaze could be its equivalent of GoPro's Session.

3. GoPro's inventory: $289.5 million
Speaking of the Session, that little camera caused GoPro's inventory to surge 89% annually to $289.5 million, its highest level ever, last quarter. That steep rise explains why GoPro slashed the Session's price in half, why the devices appeared on flash sales sites during the holidays, and why CEO Nick Woodman appeared on home shopping network QVC in a desperate bid to unload them.

We see a similar problem with Fitbit's inventory. Last quarter, Fitbit's inventory rose 140% annually to $276.1 million in preparation for the holiday shopping season. While initial reports regarding Fitbit sales have been positive, we won't know just how many devices were sold until it reports its fourth-quarter and full-year earnings. Even if Fitbit clears out a lot of its inventory during the holidays, the Blaze, which could be a tough sell, might cause that figure to climb again.

4. GoPro's P/E: 9
As of this writing, the market values GoPro at just 9 times trailing earnings. Fitbit still has a P/E of 30. This means if the market is tossing GoPro and Fitbit into the same basket, Fitbit shares could plummet to the single digits before matching GoPro's valuation.

Fitbit's bottom-line growth hasn't been encouraging, either. Last quarter, Fitbit's GAAP net income fell 33.5% due to the recall of the Fitbit Force and stock-based compensation. On a non-GAAP basis, net income more than doubled, but its gross margin fell from 53.9% to 48.3% due to higher R&D and marketing expenses. Since Fitbit will need to spend more heavily to fend off its potential rivals, those margins will likely decline and impact its earnings.

The key takeaway
Fitbit looks troubled, but not all hope is lost. It might carve out a niche in sports performance devices, and its corporate wellness partnerships might boost enterprise shipments. Nonetheless, investors shouldn't dismiss its similarities to GoPro, since it could follow the action camera maker down the same path of plummeting sales.

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.