Shares of cable giant Comcast (CMCSA - Get Report) were on the rise recently following news that the company is branching out into offering cellphone service. But before investors get excited about new competition in this space, they should understand that Xfinity Mobile doesn't appear to be without limitations.

Back in the fall of 2016, Comcast announced that it would launch a wireless mobile service. This decision had been awaited for roughly five years, ever since the company entered a joint venture with Verizon (VZ - Get Report) to develop technology to integrate wireless services and products.

In December 2011, then-Comcast Cable president Neil Smit (who is now vice chairman of the company) said, "These agreements, together with our Wi-Fi plans, enable us to execute a comprehensive, long-term wireless strategy and expand our focus on providing mobility to our Xfinity services."

"Long-term" may be something of an understatement for the five years it's taken Comcast to get its mobile act together. And the landscape for mobile carriers has somewhat changed.

Back in 2011, Verizon and AT&T (T - Get Report) dominated the space, but in the intervening years, T-Mobile (TMUS - Get Report) and Sprint (S - Get Report) have grown their services and customer bases to become nearly as big. And, as Fortune points out, mobile carrier revenue growth, industry-wide, has slipped from 6% in 2011 to 2%.

That makes yet another competitor in this hotly competitive space look less attractive. But it's not the only caveat investors should have about Xfinity Mobile.

A big drawback to the service is the fact that it will be available only to subscribers in Comcast's current Xfinity cable coverage area -- only 39 states and Washington, D.C. Although Xfinity Mobile subscribers will be able to use their phones when out of this range, it limits the market for the carrier to only 59 million people. Compare that to Verizon's subscriber count -- nearly 146 million in the fourth quarter of 2016.

If Comcast is able to convince 10% of its current cable customers over the next few years, that would make for 2.5 million subscribers -- roughly how many new users Verizon and the other major wireless carriers pull in each quarter.

Although on the surface the pricing for Xfinity Mobile looks attractive -- $65 per line for unlimited data or $12 per gigabyte -- when compared to other carriers, the deals aren't all that hot.

For example, Verizon's unlimited data plan begins at $80 for one line but then caps off at a deal of $180 for four lines. For a four-phone household, Comcast, which won't offer "package deals," would be $80 more expensive per month. And Sprint, which just announced a $120 plan for four lines, is cheaper still.

For users who don't blast through much data, the $12-per-gigabyte a la carte option looks more appealing. But then again, Comcast is not going to exempt its own services or apps from the data allowance.

And meanwhile, the wait continues... Although we now have fleshed-out details of the service, there's no set launch date yet.

Although the news has been anxiously awaited, we'll see how many customers actually sign up to this less-than-perfect cellphone service. But even then, it's questionable whether this tiny new carrier service will make much of an impact on Comcast and its stock.

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The author is an independent contributor who at the time of publication owned none of the stocks mentioned.