On an Alabama day so oppressive that the sweat pools on your face in the shade, Alex Flachsbart talks almost too rapidly to understand and drives around central Birmingham with similar velocity. Every few minutes, he pulls over to expound on a victory: neglected public housing, a long-empty factory, a crumbling department store, all being transformed into shiny apartments or airy office and retail space.

“This was one of Birmingham’s white-whale buildings,” Mr. Flachsbart said of a former Red Cross office that had been renovated into 192 rental residences. The development happened with the help of a powerful tax break created in 2017 to lure investors toward poorer neighborhoods, an idea championed by Democrats and Republicans and cited by former President Donald J. Trump as among his proudest economic policy achievements. (“One of the greatest programs ever for Black workers and Black entrepreneurs,” he called the incentive in an appearance this week at a National Association of Black Journalists conference.)

But the relatively low-income areas covered by the incentive, known as opportunity zones, didn’t benefit equally. On Mr. Flachsbart’s tour of new projects in downtown Birmingham, the stops dry up in the historically African American northwest quadrant. There, developable lots and vacant buildings haven’t received as much of the capital flowing toward the buzzier parts of downtown.

“O.Z. was a nudge there because it was already at a tipping point,” said Mr. Flachsbart, who has put together several of those deals as chief executive of a nonprofit organization called Opportunity Alabama. “There is a wall at about 17th Street.”

Birmingham and the rest of Alabama are a window into how money has and hasn’t soaked into the ground designated as opportunity zones over the past six years. Congress is taking a closer look as it considers extending the incentive, which expires in 2026 along with most of the 2017 tax law.