Anthem Inc.'s purchase of Cigna Corp.—the biggest deal, by far, in the health insurance industry — poses rewards and risks for employers, health care benefits experts say.

The $54.2 billion acquisition will create the nation's biggest health insurer with 53 million health plan enrollees putting Anthem well ahead of UnitedHealth Group Inc., which has 46 million enrollees.

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On one hand, the mega-sized insurer that will emerge will have “more leverage in negotiating rates with medical providers,” says Jim Winkler, global chief innovation officer for Aon Hewitt in Norwalk, Connecticut.

From the employer side, the deal can be “good news,” if the much larger Anthem can “cut better deals with medical providers,” concurs Dave Osterndorf, a partner and chief health care actuary at Health Exchange Resources in Mequon, Wisconsin.

Adding Cigna's book of business “may rebalance provider negotiation leverage in Anthem's favor after years of provider consolidation that has gone pretty much under the radar screen,” says Brian Marcotte, president of the National Business Group on Health in Washington.

While the deal will result in much greater clout by Anthem in negotiating rates with providers, it also will give employers fewer health insurer choices, decreasing their ability to win better rates from insurers, some say.

“Large employers will have concerns about the merger between Anthem and Cigna because employers will be left with only three major insurers who can support large multi-state employers on a nationwide basis,” Mr. Marcotte said.

In addition, the deal, which the two insurers said is not likely to close until the end of the second quarter of 2016, means the insurers during that time will be more focused on integration than innovation.

Others say it may be too soon to assess the impact of the deal and other recent ones, such as Aetna Inc.'s proposed $37 billion acquisition of Humana Inc.

“We will have to see how things evolve,” said Michael Thompson, a health care consultant and principal with PricewaterhouseCoopers L.L.P. in New York.

The deal brings together two insurers with different strengths. For example, Cigna is a big player in the dental, life, disability and expatriate health insurance markets while Anthem is known for its strength in the Medicaid and small and medium-size employer market.

“This would be an integration of complementary organizations,” Mr. Winkler said.

Under the deal, Anthem will acquire all outstanding shares of Cigna in which Cigna shareholders will receive $103.40 in cash and 0.152 Anthem shares for each Cigna share.

Fitch Ratings said Friday it has placed Anthem's ratings on its rating watch negative list. That action, Fitch said in a statement, reflects the negative effect Anthem's acquisition of Cigna could have on Anthem's financial leverage metrics and potential operations and earnings disruptions that could arise as Anthem integrates Cigna.

Longer term, though, Fitch said the combined companies “will benefit from enhanced size and scale, a characteristic that Fitch has viewed as increasingly important given pressures placed on profit margins” by the health reform law.