the main downside as a developer is it can be a little more work to set up Prerender depending on your hosting setup (although Netlify has just a checkbox to enable) to allow Googlebot to view the page JS-free. I've been playing around with some ideas to eliminate that requirement, however.

as an end user, the only downside would be the typical SPA ones, such as the client needs to render the site with JS like you mentioned (except in cases where we use pre-rendering).

here's an example site built with Routerino if you'd like to experience what it is like in usage: https://virtourlimited.netlify.app/