Mainly that Spock is much lighter weight and really focuses on just configuration management and stateful ness. Hydra has all these crazy bells and whistles (Ray integration etc) that could be useful for certain things but kinda starts meandering from the original purpose of configuration management imo. Hydra is great and if it works for you then use it. We built Spock internally when I was at Fidelity because Hydra didn’t exist… just so happens that FB/Meta was doing the same thing at the same time so both libraries end up covering a very similar usage space

As the developer of Spock (posted in another comment) -- OmegaConf is also an awesome choice and super useful. I'd suggest checking it out too!

You can go even closer to metal and use the attrs library as well (https://www.attrs.org/en/stable/)

second favorite:

https://fidelity.github.io/spock/advanced_features/Post-Hooks

Basically lets you do any validation necessary on your configs. Spock provides some basics (greater than, within bounds, etc) but it's totally up to the user via any simple asserts or validation functions a user wants to write.

This being my favorite hidden one:

https://fidelity.github.io/spock/advanced_features/Evolve#maintaining-cli-and-python-api-configuration-parity

Most of the cool stuff is buried in the docs under advanced features :-)

https://fidelity.github.io/spock/advanced_features/Composition

(full transparency I'm the author/maintainer/core-developer. I know the docs need a re-org to surface more of the useful features)

https://github.com/fidelity/spock