Flarum is highly extension driven. The core of the Forum only contains the most basic functionality, whereas any added functionality, even stuff you would expect from any forum, is added as an extension. For example, the Tags extension is added as a bundled extension with Flarum that can be disabled.
There are really no disadvantages to adding your features this way. From a development standpoint it allows the forum to stay lean and performant, as functionality is added in a very modular manner via extensions. Performance for most extensions is not an issue, but of course it depends what the extension does. Extension developers get no access to your forum, as long as you do not install shady extensions. As a forum administrator you will have to look into what you install, but same thing applies on your local machine.