I don't think this is actually as big of a deal as it's often made out to be. Metal is quite similar to DX12 and Vulkan, so anybody who's worked on either of the latter probably won't have much trouble with Metal.
Metal is probably the best designed graphics API I have ever worked with — it's very compact, simple to understand, convenient to use and very powerful. But converting from DX12/Vulkan might not be as trivial as you make it sound. First of all, Apple uses a different shading language (so you need to take care about translating your shaders, either automatically or manually), and there are some big changes in how certain stuff works (for example, the tessellation pipeline setup is completely different).
Also, some of the Metal's more interesting features exploit the unique architecture of Apple's tile-based deferred renderer. Like direct control over on-chip cache, memoryless buffers, fully programmable blending. These are the things that make Apple GPUs shine as they allow one to implement some advanced rendering techniques more simply, and with better performance.
However, that ease of use comes with performance hit for translation layers from Vulkan/DX12 to Metal which is significant, as Metal is by far not "as close to the metal" as the other APIs.
That was the case for the initial Metal (it was basically DX11 with user-friendly interface), but Metal 2 and later... I mean, you can implement more or less arbitrary data structures on the GPU with pointer chasing and everything. And you can use the GPU to generate and submit complex draw calls (there was this neat demo in 2019 WWDC where they generate an entire landscape on the GPU, with instanced trees and everything, just from a height field). Programmable MSAA, efficient sparse textures, fine-grained resource locking, manual memory management (with aliasing), rasterization order, GPU SIMD lane access (what Vulkan calls subgroups), multi-GPU synchronization, ray tracing, you name it. Also, I am not sure whether DX12 or Vulkan allow you to bind resources directly from the shaders (didn't find any info) — with Metal this is possible (resource bindings are just pointers that can be freely manipulated). Plus, this year they are adding function pointers to shaders.
In a nutshell, modern Metal is indeed very close to metal, and it is especially close to Apple GPU metal, as it allows you to directly control the fast on-chip tile cache. And it does all that with a compact and user-friendly API (looking at you Vulkan). It can also do some hand-holding like memory management or resource tracking, but if you don't want the overhead, you are welcome to turn it off and take care of all the low-level stuff yourself.
