Hi João, Vasco,
Reactive apps work as Single Page Applications, where most of the content is created by JavaScript in the browser. Some crawlers, like google search engine, work correctly with this (they use browser-like JavaScript engines to run the code on the page on their side). Others, like Facebook (which ironically created React) do not yet support that.
At OutSystems we are currently investigating the possibility of having SEO Friendly URLs available in reactive apps. We'll also take a look into this scenario to see if there is any possible workaround for these crawlers shortcomings, although at this point in time we can't promise anything.
Cheers,
Tiago Simões