While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of GoPro (NASDAQ:GPRO) fell 5% on Thursday after Vertical Group initiated coverage on the high-definition personal camera technologist with a sell rating.

So what: Along with the bearish call, analyst Robert Lopez planted a price target of $28.50 on the stock, representing about 34% worth of downside to yesterday's close. So while momentum traders might be attracted to GoPro's post-IPO price strength, Lopez's call could reflect a sense on Wall Street that the risks surrounding the company's growth trajectory are being largely overlooked.

Now what: According to Vertical Group, GoPro's risk/reward trade-off is rather unattractive at this point. "GoPro's shares have appreciated nearly 80% following the recent IPO, and present valuation, in our view, credits GoPro handsomely for a Content Monetization initiative we deem fraught with risks, while concurrently ignoring both decelerating Revenue Growth and a range of competitive threats we deem salient," said Lopez. "Our Fair Value assumes GoPro's shares trade for 25x our CY '15 NG EPS estimate, yielding our initial Fair Value of $28.50." When you couple GoPro's still-questionable competitive moat with its red-hot stock price, it's tough to disagree with Vertical's cautious stance.