A camera's low-light capability has two parts:
* how it operates in low light,
* and how are the results (high-ISO).
Both aspects are highly subject to individual taste.
I have shot Leica M for 2+ years now and am too accustomed to the non-dimming optical viewfinder and now every EVF I have tried, fails to refresh fast enough in dark conditions. Of course, can you blame the camera? A fluent refresh rate at 60 Hz would require a shutter speed of 1/100 or 1/120 and that's a tall order with a slow lens, for example a consumer zoom.
For me, choppiness just kills the enjoyment. For someone else, the brightened view to help nail the composition even at the expense of choppy refresh, will be a gamechanger.
AF performance is another place where camera's low-light performance shows big-time.
My Leica M240 wins hugely in low-light operation because its viewfinder never dims much, despite how slow a lens I mount on it, and its AF performance doesn't weaken at all.
M fails in the second category, namely the banding noise that pops up around ISO 6400 (or equivalent) is very jarring indeed. Do I need to shoot at high(er) ISOs? Sadly I think this is something I actually would need. But thanks to operations, the 2012 M still remains my best low-light camera.