Beginner question - When to return Uni from resource and when to not?

[cheetyrooroo]

Thanks for the nice framework. When I started playing with it I just returned normal data types from my resources. But I realised after reading the docs that you can return a Uni and make the request run on the event loop thread (plz correct me if I'm wrong) which sounds like it will be more efficient when more requests come.
But we should also not do anything heavy on the event loop thread.
So as example if I have a resource that will

1. fetch data from a 3rd party api with a rest client
2. transform data a little
3. store to a db with panache

If I want to squeeze the most performance out of Quarkus:
Should I be making my resource return a Uni?
Should I make the rest client method return a Uni? (Looks like it already runs on event loop without it?)
Should I use Hibernate reactive?

What's some cases where I shouldn't return a Uni ever? Just little confused 😊