In general CUBA is totally calable of building these kinds of enterprise systems. The framework itself does not make any practical restrictions on the amount of entities / screens.
In fact for most backoffice type applications the trade off made by vaadin / CUBA to favor developer efficiency over e.g. statelessness plays out particularly well in these scenarios.
Normally for bigger applications having strong architectural boundaries between subdomains in place helps long term maintainability. If you want to build all subdomains with CUBA, you can choose application components to isolate the different parts like policy lifecycle, claims, insurance accounting, reinsurance etc. into different buckets.
When deploying you can either choose to put all sub domains (app components) into one deployment monolith (one CUBA app) or deploy it up in different CUBA apps. Same goes for DB of the subdomains.
So, bottom line: nothing prevents you from doing it.