It replaces my LaTex with unicode symbols that often look shitty, and are usually the wrong size. Also, it sometimes never rendered my equations. Don't let this totally dissuade you, I never really looked into it (switching to mathjax was easier) but I just remember that "out-of-the-box", a lot of things were broken.
fwiw, I haven't had this experience at all. KaTeX has been extremely easy to integrate into my projects with a simple katex.render(my_math_str). Perhaps you had some string encoding issues separate from katex?