The mechanism by which chemical grouts stop leaks is based upon basic textbook polyurethane and acrylic chemistry. It may seem a little mysterious because most of the reaction takes place out of sight within or behind the concrete structure, but the reactions can be duplicated in the laboratory for doubters to see. As for durability and permanence, the oldest of the chemical grout manufacturers have documented case histories that are now 50 years old. I would call these permanent repairs when compared to some leak seal alternatives.
It is true, however, that for every success story I offer, someone will bring up an instance where chemical grouting did not work. Upon careful examination, these failures are almost always due to the products being incorrectly specified or installed. The biggest problem I have encountered that leads to premature failures occurs when the contractor fails to pump grout into a crack or joint until total rejection occurs. That is the point where the crack will accept no more grout. Correct product selection and installation in consultation with the manufacturer are essential to a successful long-lasting leak sealing project.