It does the job well about 98% of the time. The few times it has issues are with websites that already have built-in "dark modes," but you can easily have the add-on remember not to enable on those sites by clicking its button in the toolbar. It does affect load times of websites, but not by a very significant margin. Sometimes UI elements on certain websites become obscured but can be easily fixed by turning off the add-on for that website.

Overall, the add-on does exactly what it needs to do, with near perfect execution. It's an essential add-on for me now along with uBlock Origin.