I think we have to be careful not mix terms.  There are 2 different rendering engine types, Biased and Unbiased. (there may be more I'm not aware of, btw, not claiming to be an expert here)

There are 3 different methods that render engines can use to actually do the software computations on your computer to render an image.  It can use the computer's CPU to render, or it can (if you have a good enough video card) use the computer's GPU to render, or in the case of some render engines it is possible it can use both the CPU and the GPU working together to render the image.

Some Unbiased render engines (not all) and probably some biased render engines too can render using the computer's GPU.  Most render engines whether biased or unbiased (though not all) can use the computer's CPU.  There is at least one render engine which claims it can use both the CPU and the GPU to render (Arion 2, which is an unbiased renderer).

So what I'm saying is there are 2 different conversations/debates to be explored here.  Whether to use a Biased or an Unbiased renderer is a different conversation than whether it is more advantageous to use a render engine that uses a computers GPU to render rather than it's CPU, or whether it's better to use a render engine that combines GPU and CPU to render.

Since Embree is a library of ray tracing kernels that can be integrated into existing biased render engines to increase their speed and quality of their performance (I have no idea whether Embree could be integrated into an unbiased renderer too, does anyone know? My impression is no, but I could be wrong), and since Octane is an unbiased renderer that renders using a computer's GPU, to compare/contrast the two would (sort of) be comparing the biased vs unbiased, and (sort of) be comparing the CPU vs GPU approach. 

At least that's my understanding (and I don't claim to be an expert).