Worksheets certainly are. But good homework, even if it's challenging, is what makes a reasonably fast-paced course even possible. In a well-paced university course you're typically spending proportionally several times as much time working on it out of class than you are in class. Then class time is both preparation and catch-up, similar to office hours.
This was true of my most demanding humanities courses (sometimes reading 100 pages a week directly from academic journals, not easy reading) as well as my most challenging math courses (group theory, ring theory). Once the pace gets fast, there just isn't enough time for you to learn everything you need to inside the classroom anyway.
And in those classes, where homework was really essential for learning at the required pace and depth of mastery, my instructors didn't even need to factor the homework into my grades at all. In some of them, we could get "feedback" on homework but it was never officially recorded in our grades... and yet, anyone who didn't do it would fail the next test. If homework doesn't have that characteristic, it probably doesn't need to be assigned at all.
If "flipped classroom" means that students are expected to do all of their homework in class, then indeed it'll feel like a waste of time to many of the smarter kids, and it will also just be unfeasible for advanced courses (which theoretically should be most courses in a university, though it currently isn't). But if it means "we don't even have time to lecture you on every single thing you need to learn, therefore you must arrive already having done the reading and the exercises, and we'll use this time to help clear up misunderstandings"... that's already how classes for grown-ups are in universities.