The internals of STDlibs tend to be nasty, but externally they're designed to be clear. The problem here is externally these interfaces are extremely messy and hard to follow, i.e. the abstraction doesn't seem to work.
Scala's STDlib might be more powerful, more honest and more consistent, and also might be striving to be DRY for the implementors.
Personally, I feel that Scala's Collections goes overboard in hitting the above goals, and the resulting signatures are too verbose. And this increases the cognitive load for the subset of people that need to bother about it (performance tuners, architects, etc). But that's not a fault of the language per se.