The low-level graphics APIs like Metal and Vulkan allow for much better performance, but they are much harder to use and require more work from the developer (hence, they’re usually used only by game engine makers). Higher-level graphics APIs like OpenGL are less efficient and have lower peak performance, but are easier to use for individual projects and have the benefit of having existing functional code (no need to rewrite a working project).
Also, OpenGL (and its mobile and web variants OpenGL ES and WebGL) are very portable (macOS, Linux, Windows, iOS, Android, both and native and in browsers), while Metal is macOS/iOS-only.
Maybe I'm biased - every time I looked at OpenGl code I shuddered and ran away to a higher level framework (I'm excluding shader code from this - that's concise enough for me not to mind getting that close to the metal)
Vulkan, you need to know exactly what you are doing. There are little to no handhelding. You are trying to squeeze out every last 10% of performance in exchange for lots more development time. You need to write a hell a lot more code to do something what you previously thought were simple.
OpenGL is higher level that should be compared to Direct3D 10, not 11. As a matter of fact I will go about saying compare to Direct 3D 9. And unless you are a OpenGL zealot, any sane game developers would have told you Direct X 9 already exceed OpenGL in most way.
Metal is offering what most of Vulkan can do and making it even easier then OpenGL.
Honestly I don't get / understand why all the backlash. OpenGL is deprecated, and it simply means Apple won't be updating OpenGL anymore. Stop asking every god damn year. They have been shipping other deprecated library and API for years! OpenGL is developed in such a way that no one really cares. And designed by committees for maximum backward compatibility. And if you want your App on iOS, you will have to use Metal anyway.
That's why I wasn't sure what Metal is offering instead.