A merger between Sprint and T-Mobile could pay big dividends but would be “operationally daunting,” according to Moody’s Investors Service.

Speculation of a merger between the two U.S. wireless operators has continued to heat up in recent weeks, due to both the end of the quiet period following the incentive auction of 600 MHz airwaves and the likelihood of a lighter regulatory stance of Donald Trump’s administration compared to the Obama White House. And the companies have made no secret of their eagerness to engage in discussions.

“A merger between Sprint and T-Mobile USA could ultimately extract $3 billion or more in annual run-rate synergies from operational expense savings,” Moody’s Senior Vice President Mark Stodden said in prepared remarks. “But integration could take three to five years to achieve, and if it stalls or is derailed by operational missteps, the downside is catastrophic.”

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Integrating the carriers would likely take “significant time,” Moody’s said, giving Verizon and AT&T an opportunity to grow their market share. T-Mobile could see increased churn as Sprint’s network gradually shutters, and “operational missteps” could prove costly to T-Mobile’s service reputation. Also, T-Mobile could be forced to sell spectrum “and concentrate market power” with its larger rivals if the integration isn’t executed well, Moody’s said.

T-Mobile and Sprint appear to be a good fit on multiple levels. Sprint has a bundle of valuable 2.5 GHz spectrum but faces severe financial constraints, while T-Mobile has solid finances but lacks high-band spectrum. And joining forces would create a consolidated market with three major carriers of similar size.

Whether federal regulators would approve such a deal remains unclear, however, Moody’s noted. SoftBank spent more than $20 billion to acquire Sprint in 2012, and the company had hoped to acquire T-Mobile as well, merging the carriers to take on Verizon and AT&T. That effort was dropped when U.S. regulators indicated they were opposed to a merger, however.

“Concessions to regulators likely would be needed for such a transaction to go ahead, however, given the benefits from prior rulings against it,” Moody’s wrote. “Both Sprint and T-Mobile USA have a track record of price disruption, and would likely be willing to commit to a regulatory mechanism that ensures continued price competition, especially in light of the spectrum advantage.”