Why is using Combiner function not preferred in shuffling

Question

Though a combiner function reduces the shuffling, it is not a preferred method and merging keys are used instead. My question is, why is it not preferred?

Answer 1

Combiner function is not preferred because when a Combiner function is used, there is no way of controlling how many times and if at all Combiner will actually be used.
The default Shuffle & Sort mechanism is based on alphabetical sorting and hash shuffling of the keys and is preferred over Combiner function.