One problem with WGSL is that the binding annotations (e.g. @group and @binding) are closely tied to the WebGPU resource binding model via BindGroups, but doesn't match other 3D APIs (except Vulkan's descriptor sets maybe). Don't know how Slang solves this problem though (e.g. if it is HLSL compatible it might suffer from the same problem - just with D3D's resource binding model).

WGSL will also always only cover the WebGPU feature set, which is very conservative by definition (e.g. no modern features like mesh shaders, raytracing, etc...). Again, I don't know how much of this Slang provides though.

It's not just syntax, it is semantics as well.