Can espresso be made at cruising altitude?
The pressure differential between the boiler and the cabin would be very large. I fear that this would result in the espresso machine being compromised.
  


The pressure in the espresso machine is 9 bars, 14 in a good one. The difference in outside pressure is 0.3 bars. That is comparably tiny value that does not make any practical difference.


  
  Water would boil at a much lower temperature (198°F / 92°C) resulting in the machinery not working correctly.
  


Coffee should not be made with boiling water anyway. 198°F / 92°C is sufficient for making coffee.

Besides, if the boiler was pressurized, the water would not boil until way higher temperature (it isn't, but it does not heat the water above boiling point at that pressure).


  
  A sudden depressurization of the vessel could certainly result in a dangerous pressure differential resulting in explosive decompression.
  


The pressurized content is water, not vapour, so it would not significantly expand if the machine cracked. After all, its pressure is relieved as part of normal operation (when the coffee drips to the pot) and it does not expand.

And its amount is tiny compared to the total volume of the aircraft anyway. Very tiny, because the pressure is only in the pipe from the pump to the ground coffee.



But that all assumes the coffee machine is espresso, which most of them are not. Usually they will be just filter machines where the water flows by gravity at ambient pressure or perhaps at some pressure, but much lower than in a real espresso machine. I believe some airlines do advertise they serve true espresso though.