one aspect on the complexity of software projects is a complex graph of dependencies (I'm not thinking only to other packages, but class/module dependencies inside a single package/library).
Functional programming encourages to use really tiny and precise dependencies (i.e. functions) instead of collections of them (e.g. classes).
This makes the dependency graph more complex. Hence managing and organizing the dependency graph becomes harder.
Could this be a reason why in functional programming projects I tend to see less code organization than in OOP ones?