I would suggest that a steam plant would generally be more reliable than diesel, but if anything did go wrong in a steam set-up it was often a tow-home job.
I remember during the sea trials of the RFA Olna, the manoeuvring valve spindle fractured whilst she was going full astern, fully ballasted down to her marks. Her draught was too great to enter the Tyne, and her cargo pumps were steam powered, so we careered out of control across the North Sea, chased by a posse of tugs, until we could lighten her, turn off the boilers and be towed home.
Olna had a new Pamatrada design that had very small blades in the first row of her HP turbine. A pair broke-off and went through the machinery. Not nice. We removed the first row and it did not seem to make any difference to her performance. So we removed the first row in the machinery in her sisters. John Brown’s built QE2’s turbines to Pamatrada plans and she had the same problem on her shake-down cruise and again on a transatlantic voyage.
QE2 managed to get oil in her feed water and wipe out all of her boilers off Bermuda. Towed into port again. She had further machinery problems in the Med, just before her conversion to diesel-electric propulsion.
Other high profile steam propulsion systems became unreliable. Examples are Northern Star and Pacific Sky. I think the modern multi-engine diesel-electric or gas-turbine systems are the most effective because the spare capacity will allow an engine to be shut down for repairs, but of course relatively few ships can enjoy that luxury.