I took sometime messin' about with Customizer. I do not know why any coder would say its hard to use. Its easy peasy and I might say rather well suited towards its task. Having not experienced lots of themes playing with them, exactly how many features does one care implement?

My only experience with this is one Envato theme (a top seller I guess?) where I had to fix the Emoji JS for friends. That was simple enough. But the theme, OH MAY GOSH what a mass of goo in as far as code.

In as far as add_query_arg these functions in general. Its never a great idea to be manipulating URL's when SESSION data can be used instead for most purposes. If one is modifying URL's to for example try increasing search engine spidering based on say their plugin its just not a great idea though at least that has purpose.