It depends on the crack. For a non-moving crack in a poured wall, structural epoxy injection bonds the concrete back together and restores the wall's integrity. For an active or leaking crack, polyurethane injection seals the leak flexibly. But if a crack is caused by ongoing movement, injection alone is not enough; the movement must be stabilized.
More detail
Crack injection is a legitimate repair, but only for the right kind of crack, and understanding the distinction prevents wasting money on a fix that will not hold. There are two injection materials for two purposes. Structural epoxy injection is a rigid, high-strength resin pumped into a crack in a poured concrete wall; once cured it is stronger than the surrounding concrete and effectively welds the wall back into a single structural element, restoring the integrity the crack interrupted. It is the right choice for a crack that is not actively moving, where the goal is to re-bond the concrete. Polyurethane injection is a flexible material that expands and seals; it is the right choice for a crack whose main problem is water intrusion, because it stops the leak and can flex slightly with minor seasonal movement without re-cracking. The critical caveat is causation. If a crack exists because the wall or footing is still moving, settling, bowing, or being pushed by soil pressure, then no injection will hold, because the movement will simply crack the wall again next to the repair or reopen it. In that case the injection is treating a symptom while the cause keeps working, and the real fix is to stabilize the movement first (piers for settlement, stabilization for a bowing wall, drainage to relieve pressure), after which the crack can be injected. This is exactly why diagnosis matters and why we evaluate the cause of a crack, often with an engineer for anything structural, before recommending injection. A contractor who injects every crack without asking why it cracked is selling a temporary patch. Done on the right crack for the right reason, injection is a permanent, cost-effective repair, typically $500 to $1,500 per crack.