The definition of "low light" is critical to these conversations, and because it is so subjective, the debate is almost meaningless. In my case, I'm sitting here in my den, appx. 12' by 24', with one 100 watt tungsten bulb in the corner lamp. The lamp is behind me and when I focus on the opposite wall, containing well filled book cases of aged cherry, my 5D3 clicks to focus quite easily and without any delay. I call this low light, and have no problem at all. I cant imagine a faster or sharper focus! And when I take the time to set the custom WB after use of a gray card, the images come out perfectly.
Perhaps my standards are lower than others, but I couldn't be more satisfied with my 5D3.