No such thing... that's what happens when you let marketing into engineering.

Over time using the trainer will mean the chemical composition in the fluid solution will break down, and will be less "thermodynamically neutral". Temperature stability will only be within an upper and lower range of temperatures, and speeds. Outside those ranges the fluid is not "thermodynamically neutral", and those ranges will become narrower due to use.

That said, the trainer will be consistent day-to-day, and workout-to-workout. However, if you were to plot the in-ride power against a strain-gauge-based power measurement, you'd find that the in-power is less accurate season to season.