Single word for "never fails"
1- Inerrant
2- Unblemished
3- faultless
4- Indefectible
5- Immaculate
6- Impeccable
7- Flawless
8- Infallible
Yes:
unnerring: Committing no mistakes; consistently accurate.
infallible : 2. Incapable of failing; certain: an infallible antidote; an infallible rule.
foolproof: 2. Effective; infallible: a foolproof scheme.
unfailing : 3. Incapable of error; infallible.
I like:
- surefire
- reliable
- rock
- guaranteed