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.

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