120fps is about 8ms between refreshes, so has the potential for causing less lag than raising the SLR mirror.
Even if the refresh rate is high, there might be a long lag while the video image is being processed. Lag and refresh rate are not directly related at all.
My experience is from Fuji X-Pro1 and X-E1. There might be cameras with less lag.
Quite clearly it is now cheaper to make video viewfinders than optical ones, and they are also smaller. I see no reason to start using them until they are also better.