Your point about moving parts is good, it is absolutely the place to look for possible failure. That said, it is also possible to build such things in a durable way. For example, at the science museum in London there was a display with a Stirling engine that over 75 years old and still works fine. I expect the challenge will be to keep the materials within the range of temperatures and pressures that do not cause structural change in the material.
Has that stirling engine had zero maintnence in the last 75 years? If so that is quite impressive and gives much more confidence that NASA can pull this off with the benefit of three quarters of a centruty of technological advancement.
