Roc language can get away with using only Result because of its novel error handling. It will "infer" the error types for functions. Normally having a lot of error types is kind of cumbersome to handle, but inferring error types makes it easy to add safe error types, then why not replace all Options, etc with a single type as well?

If you are interested more there is a good YouTube from five months ago by Richard Feldman: https://www.youtube.com/watch?v=7R204VUlzGc error handling starts from 28 minutes, at 42 minutes you can see the inferred error types, which is very neat.