Over the years our team has gotten good at grooming and splitting up tickets, at estimating individual tickets and keeping a steady Sprint velocity.

Each 3 week Sprint we deliver what the team commits itself to.

However on a macro level we fail spectacularly. Initial Roadmap estimation of how long new features take are way off. A new feature should be ready in 3 sprints.. make that 6. This milestone should be reached in 2 sprints.. Make that 4.

It often comes down to hidden requirements that are discovered midway or prototypes that get into feedback loops with stake holders and take much longer then expected. Compared to those problems, we Rather seldomly we hit unexpected technical problems that prolong a project (but of course this happens too)