SimpleController contains the logic for access to the
ci ContainerInterface, so if you don’t extends it, you won’t be able to use any services : https://github.com/userfrosting/UserFrosting/blob/develop/app/sprinkles/core/src/Controller/SimpleController.php#L36
In UF 4.2, we’re adding Traits and logic to help test controllers and sprinkles, as well as documentation (coming soon !).
IMO, sprinkles should be tested agains’t the whole UF stacks if you want to make sure it implement correctly in the framework, and it doesn’t break the framework. Of course, if you have standalone classes, it won’t matter.