MoltenVK is explicitly not enough for vkd3d-proton. It lacks features that are now mandatory for acceptable performance. So lack of native Vulkan there is surely a deal breaker.
From reading about dev effort to support Mac with DXVK, there are certain things Metal/MoltenVK doesn't support directly. So current compatibility requires more hacking as well as the additional layer of translation to hurt performance.
Metal is fine, Vulkan support as well would be ideal.
I think VK_EXT_descriptor_buffer is a critical one, and there are probably more: https://www.khronos.org/blog/vk-ext-descriptor-buffer
And that's Vulkan 1.3 which MoltenVK doesn't support (yet): https://github.com/KhronosGroup/MoltenVK/issues/1776