Well that all depends on how well the features are integrated.

Often times a new feature is similar from the API perspective to pre-existing ones.

If that's the case either integration is simple, or you need to refactor the underlying API (which means that when you eventually get around to implementing the feature it (and similar future features) will integrate cleanly).

yup - and because the number feature interactions is exponential with the number of features, things get much, much worse as the number of interacting features increases.