It's not exactly a secret that Spec is not intended to be a type-like system. But it turns out, it's perfectly possible to use Spec as a building block to do so!

As for complexity, I think instrumentation (Spec's approach) is complex. People routinely struggle with it, in fact you must use an external lib (Orchestra) to fully enable it. It also slows down execution, sometimes disproportionally.

Whereas Schema simply has a global toggle that also can be overriden in a fine-grained manner. It's explicitly made to be fast and have a type-like use.

As for type definitions, honestly both are elegant and powerful; you can quite easily use arbitrary predicates as "types" with both, and compose those predicates. Ultimately Spec is better designed because it fosters namespaced-qualifed keywords which compose better.

For addressing the instrumentation problem, I created https://github.com/nedap/speced.def which uses Clojure's :pre system. I have used :pre for over a decade so I know it has an extreme simplicity. Spec can take :pre's usefulness to the next level.