I think what people might miss the most in custom engines could be fast to use editors, they are hard to make. But there are things like Tiled (https://www.mapeditor.org/)  or ldtk (https://ldtk.io/) that can be used as 2D editors for your own framework. dearImgui (https://github.com/ocornut/imgui)  can also provide a great resource to build your own tools inside the game itself.