In what sense?

wp-includes/class-wpdb.php

In the sense that it and its ecosystem is high maintenance. When I call something names I do not do that lightly, but Wordpress is probably the single piece of software that has caused me the most problems over my 20 year IT career. You simply cannot put Wordpress into a customers hands, let it run for 5 years and expect it not to blow up in one way or another.

Just today I spent three hours to manually fix a page where a customers site was defaced after they installed and then uninstalled a translation plugin. I had to write a script that manually check every single instance of translatable text there is on the website.