The problem with The Dent Act, was Dent himself. Harvey's convictions relied on him being a fair, impartial, and above all LEGAL District Attorney. When he became Two-Face, his first acts were to kill two police officers(dirty yeah, but they WERE still cops), the driver of a mob figure(and I think the mob boss Moroni himself) and kidnap Gordon's entire family to hold them hostage.
Any lawyer will eagerly tell you this is more than enough evidence to cast doubts as to the judgement and professional objective discretion a DA is required to maintain in order to perform their job. Because of that, every criminal Dent put away, from mob bosses to jaywalkers, COULD make a legal case that Dent didn't do his job properly. The BIG problem with that, was that Dent racked up 549 mob bosses or personnel....549 separate cases that need to be re-tried, all the evidence re-examined...ALL AT THE SAME TIME.
TDKR showed that Gordon compounded the problem, by allowing Dent's true nature to be hidden, and Mayor Garcia used the Dent Act to CONTINUE imprisoning and arresting up and coming mob figures. So instead of 549 separate cases...The Dent Act allowed over 1,000 individuals to be arrested and held without trial, and those who WERE tried, to be held without a chance of parole.
The Dent Act became a form of "Star Chamber Justice," where the government decides guilt or innocence based on expediency and arbitrary rulings that facilitate the needs of the government, rather than whether or not a person actually committed the crime. It was convenient for Gotham to be able to lock up the mob without evidence, without a court order. It was convenient for Gotham to KEEP the mob locked up, while the government tried to rebuild Gotham City. The surety of the STATE, was more important than justice: that, is why the Dent Act was bad.