Given Vulkan is in some ways lower-level than Metal, AIUI, surely there's a significant cost that would make targetting Metal directly a better solution for things performance-critical?
Most of the critical low level of components of Vulkan are there: multiple command queues with manual synchronization, resource heaps (although they are optional for Metal), mixed compute/render support. Mostly Metal simplifies some of the distinctions Vulkan drivers may dump on the user: for example, Metal has only one command queue type while a Vulkan driver may force the user to manage several.
The biggest hurdle I can think of is geometry/tesselation shaders. Newer versions of Metal support tesselation shaders, but they work rather differently than Vulkan's (they're based on a compute shader, instead of the Hull/Domain split other APIs use). Geometry shaders aren't supported at all.
