I'm doing dynamic web apps on a different stack but dabbled with doing web apps in Lisp and found the tradeoffs to be quite similar.

Pro: rapid prototyping and development, excellent debugging capabilities (both for your web app and network requests... can be extended to browser-side with effort), your app can have capabilities that are difficult to implement with static languages/frameworks

Con: small user community which results in limited selection of frameworks all having their own quirks/limitations, when things break you're often on your own, in dynamic languages performance often won't scale as easily or as far vertically (the recommended approach is often to be sure your design can scale horizontally)

We are using a slightly different set of libraries. CL-WHO for generating HTML and XML for sure. AllegroServe for the webserver (with zacl, a new Allegro CL compatibility layer, for non Allegro CLs).