All the time I hear from fellow devs "oh don't advertise you use WordPress, it's irrelevant to clients." Tell that to my bank account and all the jobs I've stolen from agencies who wanted to use their BS proprietary CMS.
I sell WordPress like this: when you want a rebuild in 10 years and I'm in Thailand smoking drugs with your money pile you just gave me, you'll be able to find someone to work on it no problem.
There's plenty to dislike about the WP codebase, but we all have to remember that WP was made in an era when coding reasonable software in PHP was nearly impossible. Nothing backwards-compatible can happen there that isn't going to destroy all the millions of existing sites. It's still pretty freaking easy to build a site with, especially when you compare it to other CMSes of that era.
Moreover, their development roadmap is actually really solid. I like where WP is going. I unfortunately would no longer recommend it for basic blogging due to its complexity but as a CMS, there's really not a lot of free software options that can function on that level without extensive database programming and security knowledge.