What does WordPress.org do well? Every imaginable feature can be added using themes and plugins, most of which are open source, which means free to use. This freedom means that instead of a development process and then fine tuning and bug fixing, you can upload a plugin and usually configure it in a few minutes. Wordpress is so flexible and easy to use; it's no wonder that it powers almost a third of the web.
What does WordPress.org not do well? If a client wants a lot of customisation to a theme, it can be a challenge because of potential clashes with other plugins and code. Updates can overwrite custom script. Some complex aspects to a website can be achieved more elegantly with writing in raw software code, not using plugins.
What do you wish you had known about WordPress.org before using? I used to add too many plugins, many of which weren't needed. This bloat can slow a website down. I should have stuck to a handful of important ones, then focus on content and the core business.
Have you used a competitor to WordPress.org? If so, how does it compare? I've used a few, and they all were far less flexible and had steeper learning curves. Wordpress was, and still is leaps and bounds better than everything else, unless you write custom code, and then it depends on the specific use case whether you'll end up with a better result.