Maintainability

Reason may take some time to get comfortable with, but you can think of this time as an investment in the maintenance and confidence of your future product. Although languages with gradual type systems, such as TypeScript, may be easier to get started with, they don't provide the sorts of guarantees that a sound type system such as Reason's can provide. Reason's true benefits cannot be completely conveyed within simple examples, and only really shine when they save you time and energy in reasoning about, refactoring, and maintaining your code. Put it this way; if someone told me they were 99% sure a spider wasn't in my bed, I would still have to check the entire bed because I don't like bugs!

As long as you're 100% in Reason and your code compiles, the type system guarantees there will be no runtime type errors. It's true that when you are interoperating with non-Reason code (JavaScript, for example), you introduce the possibility of runtime type errors. Reason's sound type system allows you to trust that the Reason parts of the application won't cause runtime type errors, which therefore allows you to focus extra attention on ensuring that these areas of the application are safe. In my experience, programming in a dynamic language can feel noticeably dangerous. Reason on the other hand feels like it always has your back.