It can also often lead to a dead end when the client so often doesn't know why they feel something or respond a certain why. It implies a deeper reason than the surface reason, and tends to distract clients from further exploration that they can then feel validation and control from.

And it is the "onion" question that simply leads to endless layers of whys.

It isn't inherently a bad question, and can be very appropriate for some clients to encourage deeper reflection.