Kong is written in Lua and extends OpenResty which extends NGINX with LuaJIT. Kong provides many API related features like i.e. a Developer-Portal to onboard new users for your API, unfortunately this and also some other features are not available at the open-source product and can be only used with Kong-Enterprise.
Fusio is fully open-source this means we have only one product where all features like the developer portal are contained. Fusio can also work with NGINX since it is a plain PHP application. Another great feature of Fusio is the possibility to automatically generate a client SDK for your API.