I’d say Discourse is even more customizable than Wordpress. You can craft theme components in minutes, browse through thousands of themes, make your own plugins with ease without troubling yourself about if it breaks 50 other things or even download official ones/ones made by the community.
I would argue that it is. I’ve written a dashboard that installs and manages discourse instances as a Discourse plugin as well as one to process arbitrary forms.
There are not as many plugins as there are for wordpress because it’s not been around as long. To me it’s much easier to develop in discourse than it is to maintain a wordpress site.