I disagree. GLFW has MANY bugs.. I ran into enough that I just gave up on it.
I hope you reported those bugs, so that they can be fixed. I haven't ran into any thus far.
And as for assimp, I use assimp in my engine because my engine supports modding and I want modders to be able to load any file type that they want. Think of an engine like Unity. If it used a special format, no one would use it because it would be to complicated to convert EVERY model to the proprietary format.
Unity is more than a game engine, it is a game engine and a advanced editor. There is absolutely no logical reason that the engine needs to be loading models from a number of different textual file formats. Have your assimp support built into the editor, so your modders can load assets to their heats content, and have that editor convert them to a binary representation specific to your engine, that way when someone actually distributes a game it can load the assets quickly. People don't like long loading times, and that is exactly what you will get if the engine has to parse every single model from a text or XML file.