Oddbean new post about login | logout for real, these days a 2D engine is not as hard to assemble together, there are several open-source frameworks that will speed up that process a lot. 3D is a different thing, but for 2D it's doable for 1 person only.
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.