Yeah, to add to that, I've hit bugs in blending where certain blend modes just don't work and silently render transparent black. Also, the Metal debugger crashes most of the time and tells me "file a Radar". I'm likewise dealing with reported segfaults on hardware I don't own. And there are missing features: timer queries, for instance, aren't really supported and you have to hack them up in an unreliable way, useful extensions supported by the hardware like conservative rasterization are unavailable (Vulkan has this problem too), etc.
I suspect things are better on iOS, but Metal has not been a great experience on macOS for me. There's an argument to be made for proprietary APIs if they're better than the alternatives, but not if they're worse...