What is an alternative, succinct phrase for something that is not prone to errors, i.e. the antonym of error-prone? "Error-free" is too extreme in that it suggests infallibility.
I think you want:
Reliable 2: giving the same result on successive trials
Reliability: The probability that a product will perform its specified function under prescribed conditions without failure for a specified period of time.
from APICS.org dictionary (on-line, 13th ed.)
Reliable, steady, stable, or high-quality may work, depending on the context. If that's not quite right, try those words as a starting point in a thesaurus.
What about "safer"? I know it's not specific to the pass/fail rate of a working unit, but it does imply that it's fairly good but definitely not perfect.
I like peerless for describing a human trait.