You skipped over the first part "Has no pitfalls, aka the type system is "sound" (the types will always be correct). E.g. If a type isn't marked as nullable, its value will never lie and let through some undefined value silently. ReScript code has no null/undefined errors."

E.g., if a type returned from a function is nullable then you will be forced to handle that explicitly via pattern matching I imagine, rather than being able to dereference it without checking.