My idea of client power has always been the right to hire and fire and the right to evaluate the therapist's performance. The second could be ignored by the therapist if the client offers some feedback - it never was by either of mine - but hardly the first. And ignoring the second without a good reason would be grounds for the first to me.

So if a client goes in with those two ideas of power at minimum, I'm not sure how they could be devastated.