It depends, as both front-end and back-end have large ranges of complexity. In general though, if you were to compare the most extreme case of back-end complexi

It depends, as both front-end and back-end have large ranges of complexity. In general though, if you were to compare the most extreme case of back-end complexity to the most extreme case of front-end complexity, the back-end case would be more difficult. The worst case of back-end complexity requires very in-depth knowledge of hardware, networking, distributed computing, OS internals, and then knowledge of a couple of languages and CS fundamentals thrown on top. The worst case of front-end complexity requires learning and writing tooling to make writing desktop replacement apps in a browser a less terrible experience. Tack on UI/UX design, you are still not quite at the same level of complexity. I don't mean to say that front-end isn't difficult or that it doesn't take years to master, just that back-end is even worse...