One of the good things about it is that using asynchrony is optional. If you don't have to call out anywhere to build the response, processing can all stay in the handler's calling thread. If you do, you can return a future and have the library handle the async for you.

One downside is that it is based on Jetty which isn't considered the most performant backend. A lib with a similar API but based on Netty is Jooby [1] which scores well in the Techempower benchmarks.

[1] - https://jooby.io/