Either the size of the data structures created is limited and known, or it isn't. Whether the work is done in a refcount-decrementing recursive descent or a tracing garbage collector doesn't change that part.
And a refcounted tree freeing won't necessarily run at the end of the current scope, that depends on what other work is being done currently, and the size of the tree is a subset of all allocations until last user drops it.
and yet whether or not other work is being done currently is in your control, thus predictable (unless you chose to give control out of your hands e.g. by introducing two threads racing against each other); same regarding the subset
