There is a basic-level mismatch when comparing WordPress to ProcessWire.

First -- I spent years working with WordPress, Drupal, and Joomla (aka Drumlapress, as my friend Joss notes above). All three of those systems make many assumptions about how your websites look and behave. I have now spent a couple of years with ProcessWire. It's a natural extension of the technologies powering the Web: HTML, Javascript, CSS, and PHP. In other words, knowledge of how the Web works gives you greater insights into how ProcessWire works, and vice versa.

You come to a system like WordPress looking for what to "plug in." While it's true that ProcessWire has a growing number of "modules," these are generally meant to have their source code examined and discussed openly by the general community. I dare you to try that with WordPress, Joomla, or Drupal!

Although there are a good number of plugins in the ProcessWire ecosystem, there are no "themes" or "templates." A lack of themes is not a weakness, but rather one of the most attractive aspects of ProcessWire. Like HTML/CSS/Javascript/PHP, ProcessWire does not make assumptions about how your site is designed. Themes, by definition, lock you into a list of assumptions.

It's also important to note that although ProcessWire has an "admin back-end," its real beauty and strength come from the fact that its core is a clean PHP framework. For that reason, ProcessWire perhaps is more related to systems like Yii, Laravel, and CodeIgniter than it is to the "big three" CMSs.

I leave you with an analogy from woodworking: WordPress, Drupal, and Joomla are like pre-made project kits. All the wood pieces are cut to the right sizes for you, numbered according to how they must be assembled, with pre-drilled holes. You even get a set of various sized screws, a little bottle of glue, three mini cans of paint (just enough for the project), and a set of instructions you need to follow. With ProcessWire, it's more like you have a project idea, and you jot down the sizes of all the parts and make a list of all the hardware you need, choose your own the paint colors and glue. Then you go to the store to buy it all. In the end, the project kit looks like everyone else who bought the same one. The one you made yourself takes longer, but it does more, and it's unique.