Essentially every project I've worked on in the last 9 years has been agile, most of them Scrum, and almost all of them succeeded in achieving their goals. There's a clear correlation on all of them between the skill of the project manager at applying the agile approach, and how difficult the delivery was.
Prior to that, I worked (as did most people) on waterfall-style projects, and the success rate was only about 50%.
Usually when agile approaches (kanban or scrum) fail it is because there's an unsolvable bottleneck.
Typically too small team, lacking knowledge or skill for the scope, problematic environment (development or socially) or someone is pushing "done" work causing skyrocketing development debt that quickly comes to a head - typically a prototype remains in production, switching too early to maintenance mode for business reasons.
Waterfall cannot identify those risks without huge experience. Scrum tends to exacerbate some of them. Kanban with sufficient number of phases tends to expose the problem, but not necessarily fix it.
Prior to that, I worked (as did most people) on waterfall-style projects, and the success rate was only about 50%.