I think I had this issue with an early-generation EVF (first- or second-generation, and it was an add-on unit that wasn't built into the camera), but haven't had it with current-generation cameras. If anything, the camera compensates to give you an evenly-lit image, whereas with an OVF you'd basically be squinting and having to hope that your pupils constrict enough so that you're not being blinded.
If the camera company offers full functionality for their EVF, it's the same deal with the darkness. My Fuji lacks this feature and thus represents a handicap, but Olympus have implemented a special mode that slows the refresh rate in darkness so that your preview in the EVF is always lit evenly. In pitch black the EVF might not refresh more than once every second or two, but you'll still be able to see... whereas with an OVF you're limited by what your own eyes can see (which is likely better than many EVFs operating in standard mode). It's like having night vision, plus you can still magnify the scene to adjust manual focus. It's the ultimate for low-light shooting.