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.

Nothing wrong with it if you know what you’re doing and have a strong API to power it. Most people don’t know what they’re doing and end up with a duct taped ball or spaghetti. This tendency is compounded when you’re in a hurry to get shit done.

Nothing better than having to maintain 1000 line components. I wouldn't know, but I have dumped them on other people. It made sense to me when I frantically wrote it, how bad could it be? /s

But is the server side method less spaghetti-prone? I'm inclined to believe that you just believe that because it is the way you are used to programming. A beginner will write spaghetti no matter the job, no matter the tools, no matter the method.