Nothing "wrong" with them per se, but in terms of system complexity, you're basically doubling it by having application state and logic on both the backend and

Nothing "wrong" with them per se, but in terms of system complexity, you're basically doubling it by having application state and logic on both the backend and frontend. When I work on a big SPA, I feel like I'm working on two applications and have to worry about syncing state and logic together.