A large and complex system with an API and interacts with multiple other systems that is maintained over multiple years will be killed by agile through scope creep and inconsistent implementation when there is staff turnover. People will get great ideas that break other things snd without a cohesive vision across the team, things will be missed and unfinished because people focus on their part and not the whole.

You can add the structure to keep things coherent and spend more time doing documetation up front so people can review the API and do it right the first time instead of redoing it multiple times.

Agile is great for some projects, but ataff turnover, coordination, and meeting any kind of complex external requiirements means it isn't a great fit for all projects.