In short, async Rust is more difficult to use and can result in a higher maintenance burden than synchronous Rust, but gives you best-in-class performance in return. All areas of async Rust are constantly improving, so the impact of these issues will wear off over time
As the async book says, async Rust is not mature yet. While bringing performance, it also results in a higher maintenance burden. The goal of this project is to create a simple and minimum synchronous Web framework for Rust.