Sorry but that's a silly question. There is no limit to how complex you can make anything in any development environment. The less competent a developer is, the more complex the solution will be. What is hard, what takes skill, is to find a simple solution to a complex problem.

Having said that, back end applications can be anything from a simple DB server to the control software of a nuclear reactor or factory. So there is more scope for back end development to be complex than there is for front end development. So perhaps an answer is that the back end development is more likely to be more complex than the front end development.

How long is a shortest thread?

Complexity between backend and frontend is not comparable.

No it can be very simple. For example, use ESM, preact, htm plus html, css you can make powerful frontend app, without all those fancy frameworks, tools. look at the source code of https://app.moochee.us/tryout

It's hard to say because they're both different, but I think truly great frontend work is a bit rarer because there's so many more approaches that can solve the same problem, so you need to use your a bit of artistic judgement on top of being a wizard coder.

I think the problem isn't about intrinsic complexity, but simply that the tools we have to manage the abstractions we need to do complex backend work are better understood and further developed than the tools and abstractions we have to deal with complex UIs.

nah, its fine[1]. on the frontend you have a single thread and all state is in memory. on the backend anything goes. in both domains you have to use sane, simple designs, and that can be challenging.

1. http://github.com/nathants/aws-gocljs

It depends entirely upon how you define “modern.” Even plain old CSS and HTML can be used in a “modern” way.

Yes, and it's more dynamic, too.