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.