Before you read this post, I want to say the post is my opinion. Some people may think differently from me.
I started to use Modx in 2009. It was fast learning and developing. You can transfer the HTML template into Modx easily. You should know PHP for writing the snippet. You should know HTML and CSS for writing chuck. To make the multilingual website, simply to install the add-on and do some configuration. And it is done.
For WordPress, I started to work with WordPress in 2013. It had a learning curve to compare with Modx. The template system is complex at first. Once you use to it, it will be simple. WordPress API is useful but there is a lot of them to choose from. Building the plugin and theme is not too complicated.
For plugins, WordPress has a lot of plugins to choose from. I often use the free version and they are great. For Modx, you may build your own plugins since Modx doesn’t have plugins as much as WordPress plugin. For the template, you can use any HTML templates and transfer to Modx fast and easily while WordPress, you will take more time to transfer the HTML template to WordPress template. But you download a free theme or buy the WordPress theme then customize to fit your needs. I never have any trouble with any free themes and bought themes that I choose before.
For customers view, they prefer WordPress to Modx. They say WordPress is easy to use and learn.
