  Using the fluctuation theorem supplemented with geometric arguments, we
derive universal features of the (long-time) efficiency fluctuations for
thermal and isothermal machines operating under steady or periodic driving,
close or far from equilibrium. In particular, the long-time probability for
observing a reversible efficiency in a given engine is identical to that for
the same engine working under the time-reversed driving. When the driving is
symmetric, this reversible efficiency becomes the least probable.
