Advanced Disposal Services Inc. didn’t give investors enough information about a planned $2.9 billion merger with Waste Management Inc., a May 14 suit said.

The solid waste firm should have given investors more details about the financial projections its advisers relied on to evaluate the deal, the complaint filed in the U.S. District Court for the District of Delaware said.

Advanced Disposal’s preliminary proxy statement—filed with the Securities and Exchange Commission May 10—also left out specifics on the valuation...