APIs like that are both good and bad because they let you do almost anything so make many optimisations either hard or impossible, and have a nasty habit of exp

APIs like that are both good and bad because they let you do almost anything so make many optimisations either hard or impossible, and have a nasty habit of exposing implementation details which constrain future changes to the language implementation. Even APIs such as JNI that do their best to abstract over such details tend to block a JIT's view of what is going on and end up being a real pain.