Actual complexity depends is not imposed by the framework, but by the application itself, and the amount of front-end logic tends to be lower. Yes, there is more complexity in the setup of front end code (now there are dependencies and build pipelines), but ultimately they should simplofy things especially for slighly more complex websites.

Testing is one of the things that's generally tedious in front end applications, but not inherently complex. There may be lots of config needed (e.g. for setting up and controlling a headless browser), and long turnarounds because tests are slow and shaky. But they are also boilerplatey.

> When I say modern frontend - its mostly React, state management like Redux, Zustand, Router framework like React Router, a CSS framework like Tailwind and component framework like Shadcn

AI is the best at adding standard things into standard boilerplate situations, all those frameworks just makes it easier for AI. They also make it easier for humans once you know them and have seen examples, that is why they exist, once you know those frontend is not hard.

That's just a farmiliarity thing. I've worked on project doing full web FE, mobile and BE.

It's hard to generalize but modern frontend is very good at isolating you from dealing with complex state machine states and you're dealing with single user/limited concurrency. It's usually easy to find all references/usecases for something.

Most modern backend is building consistent distributed state machines, you need to cover all the edge cases, deal with concurrency, different clients/contracts etc. I would say getting BE right (beyond simple CRUD) is going to be hard for LLM simply because the context is usually wider and hard to compress/isolate.