I noticed a lot of 1 star review mostly from people that find it difficult to use the add-on, I guess that's lack of detailed guide.
It works totally fine, here's how to use it...
(eg: in Docs) After successful installation, click on extensions >> add-ons >> manage add-ons.
click on the 3 dot (...) options on the top right of the add-on and select "use in this document".
close and click on the extensions again >> add-ons >> view document add-ons, it will open.
search for the icon you want to use, click on it, change the color (optional) then click insert.
this will insert the icon on the position of your cursor.
Hope this helps.